EP2952601A1 - Electric-resistance-welded steel pipe - Google Patents
Electric-resistance-welded steel pipe Download PDFInfo
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- EP2952601A1 EP2952601A1 EP14746008.3A EP14746008A EP2952601A1 EP 2952601 A1 EP2952601 A1 EP 2952601A1 EP 14746008 A EP14746008 A EP 14746008A EP 2952601 A1 EP2952601 A1 EP 2952601A1
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- steel pipe
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- pearlite
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 81
- 239000010959 steel Substances 0.000 title claims abstract description 81
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 41
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 34
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 17
- 239000000203 mixture Substances 0.000 claims abstract description 14
- 239000000126 substance Substances 0.000 claims abstract description 14
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 11
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 7
- 239000012535 impurity Substances 0.000 claims abstract description 6
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 5
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 4
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 3
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 3
- 229910052796 boron Inorganic materials 0.000 claims description 3
- 229910052791 calcium Inorganic materials 0.000 claims description 3
- 229910052802 copper Inorganic materials 0.000 claims description 3
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 229910052759 nickel Inorganic materials 0.000 claims description 3
- 229910052758 niobium Inorganic materials 0.000 claims description 3
- 229910052721 tungsten Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 230000000694 effects Effects 0.000 description 25
- 238000001816 cooling Methods 0.000 description 23
- 230000000052 comparative effect Effects 0.000 description 20
- 230000007423 decrease Effects 0.000 description 19
- 230000006866 deterioration Effects 0.000 description 17
- 239000002994 raw material Substances 0.000 description 13
- 229910052729 chemical element Inorganic materials 0.000 description 12
- 238000000034 method Methods 0.000 description 10
- 230000009466 transformation Effects 0.000 description 8
- 238000010622 cold drawing Methods 0.000 description 7
- PMHQVHHXPFUNSP-UHFFFAOYSA-M copper(1+);methylsulfanylmethane;bromide Chemical compound Br[Cu].CSC PMHQVHHXPFUNSP-UHFFFAOYSA-M 0.000 description 7
- 230000008569 process Effects 0.000 description 6
- 238000003466 welding Methods 0.000 description 6
- 238000010586 diagram Methods 0.000 description 5
- 238000004519 manufacturing process Methods 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 150000001247 metal acetylides Chemical class 0.000 description 4
- 229920006395 saturated elastomer Polymers 0.000 description 4
- 238000002791 soaking Methods 0.000 description 4
- 229910000677 High-carbon steel Inorganic materials 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 230000003247 decreasing effect Effects 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 230000006872 improvement Effects 0.000 description 3
- 239000000047 product Substances 0.000 description 3
- 230000000171 quenching effect Effects 0.000 description 3
- PIGFYZPCRLYGLF-UHFFFAOYSA-N Aluminum nitride Chemical compound [Al]#N PIGFYZPCRLYGLF-UHFFFAOYSA-N 0.000 description 2
- 239000000470 constituent Substances 0.000 description 2
- 125000004122 cyclic group Chemical group 0.000 description 2
- 238000009661 fatigue test Methods 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 238000011835 investigation Methods 0.000 description 2
- 229910000734 martensite Inorganic materials 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 238000005096 rolling process Methods 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 238000005496 tempering Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 238000005261 decarburization Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 229910052755 nonmetal Inorganic materials 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000001902 propagating effect Effects 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
- 229910052761 rare earth metal Inorganic materials 0.000 description 1
- 238000010008 shearing Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
Images
Classifications
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- 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
- C22C38/40—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
- 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
- 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
-
- 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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- 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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- 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
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
-
- 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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- 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
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
-
- 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
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- 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
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- 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
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- 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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- 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
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- 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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
-
- 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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention relates to an electric resistance welded steel pipe excellent in terms of fatigue characteristic.
- Patent Literature 1 discloses a hollow drive axis which is made from a seamless steel pipe as a raw material, having a steel chemical composition controlled to be within a specified range, which is excellent not only in terms of cold workability as indicated by an austenite grain size number of 9 or more after a quenching treatment has been performed but also in terms of hardenability, toughness, and torsion fatigue strength (hereinafter, also simply referred to as fatigue strength), and which realizes a stable fatigue life.
- Patent Literature 2 discloses a technique for increasing the strength of a steel pipe by using an electric resistance welded steel pipe having a steel chemical composition controlled to be within a specified range as a raw material and by performing a quenching-tempering treatment on a weld of ERW and a portion around the weld as a hardening treatment.
- an electric resistance welded steel pipe is superior to a seamless steel pipe in terms of dimension accuracy
- normalizing is not performed, there is a risk in that, since an electric resistance welded steel pipe has low toughness, brittle failure may occur in a practical use environment.
- a drive shaft since local stress concentration occurs in a weld of ERW and in the vicinity of the weld due to cyclic shearing stress and bending stress being applied, there is a risk in that fatigue breaking may occur in a short time. Therefore, normalizing is a treatment which is very important in order to use an electric resistance welded steel pipe for a drive shaft and which significantly influences the properties of a steel pipe as a final product.
- An object of the present invention is, in order to solve the problems described above, to provide an electric resistance welded steel pipe whose metallic microstructure and tensile strength after normalizing has been performed are less likely to be influenced by a cooling rate when normalizing is performed even in the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe and with which stable fatigue strength can be achieved.
- the present inventors diligently conducted investigations in order to solve the problems described above, and as a result, found that, by controlling Al content in steel to be within an appropriate range, the metallic microstructure and tensile strength after normalizing has been performed become less likely to be influenced by a cooling rate after normalizing has been performed and that stable fatigue strength can be achieved. Moreover, it was found that, by controlling the prior austenite grain size to be within an appropriate range, it is possible to increase (1) the strength of pearlite and (2) fatigue crack propagation resistance of ferrite-pearlite steel without changing the tensile strength, which results in an increase in fatigue strength.
- the present inventors manufactured hot-reduced steel pipes (having an outer diameter of 45 mm and a wall thickness of 4.5 mm), by using a hot-rolled steel sheets (coiled at a coiling temperature of 650°C) having a basic chemical composition in accordance with the steel specification SAE1541 (containing 0.42%C, 1.5%Mn, 0.0035%N) and Al in various amounts as a raw material, by performing roll forming and high-frequency resistance welding on the raw material in order to manufacture electric resistance welded steel pipes (having an outer diameter of 89 mm and a wall thickness of 4.7 mm), and by thereafter performing hot reducing on the formed and welded pipes.
- Fig. 1 illustrates the relationship between a cooling rate for normalizing and HV hardness (Vickers hardness). It is clarified that, in the case where the Al content is 0.005% or less, almost constant HV hardness is achieved for a wide cooling rate range, that, in the case where the Al content is 0.007% or more, HV hardness is strongly influenced by the cooling rate, and that, in the case where the cooling rate is low, there is a sharp decrease in HV hardness.
- Fig. 2 illustrates the relationship between the Al content and a lamellar spacing
- Fig. 3 illustrates the relationship between the Al content and a prior austenite grain size
- Fig. 4 illustrates the relationship between the Al content and torsion fatigue strength.
- the cooling rate for normalizing was 1°C/sec.
- the prior austenite grain size increases with decreasing Al content, and the torsion fatigue strength increases along with the prior austenite grain size. It is clarified that, in the case where the Al content is 0.005% or less, such an effect becomes saturated and that the torsion fatigue strength also becomes stable.
- Fig. 5 illustrates the results of the cross-section observation of the fracture portion after a fatigue test had been performed
- Fig. 5(a) and Fig. 5 (b) respectively illustrate the fatigue crack propagation situations for a material containing 0.03%-Al and a material containing 0.003%-Al.
- the crack propagation route is indicated with a white line. It was found that fatigue crack starts from the outer surface side of a pipe and then propagates through a winding path made of soft pro-eutectoid ferrite.
- the reason why hardness varies depending on the Al content in a low cooling rate region in Fig. 1 is because, in the case where the Al content is high, since the growth of austenite grains is suppressed in a normalizing process due to the pinning effect of aluminum nitride (AlN) which has been precipitated before normalizing is performed, and, at the same time, since there is an increase in the lamellar spacing of pearlite which is finally formed, there is a decrease in hardness.
- a decrease in hardness is significant particularly in a low cooling rate region, in which quenching effect is less likely to be realized, and significantly depends on the Al content (the amount of AlN precipitated) in steel.
- AlN aluminum nitride
- the C content is set to be in a range of 0.35% or more and 0.55% or less, or preferably in a range of 0.40% or more and 0.45% or less.
- Si 0.01% or more and 1.0% or less
- Si is added for deoxidation, and it is not possible to realize a sufficient deoxidation effect in the case where the Si content is less than 0.01%.
- Si is also a solute strengthening element, and it is necessary that the Si content be 0.01% or more in order to realize such an effect.
- the Si content is more than 1.0%, there is a deterioration in the hardenability of a steel pipe.
- the Si content is set to be in a range of 0.01° or more and 1.0% or less, or preferably 0.1% or more and 0.4% or less.
- Mn 1.0% or more and 3.0% or less
- Mn is a chemical element which promotes pearlite transformation and improves hardenability, it is necessary that the Mn content be 1.0% or more in order to realize such effects.
- the Mn content is set to be in a range of 1.0% or more and 3.0% or less, or preferably in a range of 1.4% or more and 2.0% or less.
- P is an inevitable impurity in the present invention
- the upper limit of the P content is set to be 0.02% or less.
- P is a tendency for P to be concentrated in a segregation part which is formed when continuous casting is performed and to remain in a hot-rolled steel sheet as a raw material of a pipe. Since the edges of a steel strip are abutted and subjected to upsetting when electric resistance welding is performed, the segregation part in which P is concentrated may be exposed on the outer surface and inner surface of a steel pipe, which results in there being a risk in that cracking occurs when secondary processing such as flattening forming is performed on this part. Therefore, it is preferable that the P content be 0.01% or less.
- S is an inevitable impurity in the present invention, and the upper limit of the S content is set to be 0.01% or less.
- the S content is high, there is a deterioration in toughness of raw material, and S combines with Mn in steel to form MnS. Since MnS is elongated in the longitudinal direction of a steel sheet to form a long inclusion in a hot rolling process, there is a deterioration in workability and toughness. Therefore, it is preferable that the S content be 0.005% or less, or more preferably 0.003% or less.
- Al is an important chemical element in the present invention in order to achieve the desired prior austenite grain size accompanied by satisfactory torsion fatigue strength, since, in the case where the Al content is more than 0.005%, a pinning effect is realized in a normalizing process due to an increase in the amount of AlN precipitated, which results in the desired austenite grain size not being achieved due to the growth of austenite grains being suppressed. Therefore, the Al content is set to be 0.005% or less, or preferably 0.003% or less.
- N is a chemical element which contributes to suppressing the growth of austenite grains in a normalizing process as a result of combining with Al to form AlN, it is necessary that the N content be 0.0050% or less in order to suppress such an effect, or preferably 0.0035% or less.
- the Cr content is set to be in a range of 0.1% or more and 0.5% or less, or preferably in a range of 0.15% or more and 0.30% or less.
- the basic chemical composition according to the present invention is as described above, and one or more of Ti, B, Mo, W, Nb, V, Ni, Cu, Ca, and REM, which will be described below, may further be added in order to increase strength and fatigue strength.
- Ti is effective for fixing N in steel in the form of TiN.
- the Ti content is less than 0.005%, there is insufficient effect of fixing N, and, in the case where the Ti content is more than 0.1%, there is a deterioration in the workability and toughness of steel.
- the Ti content it is preferable that the Ti content be in a range of 0.005% or more and 0.1% or less, or more preferably in a range of 0.01% or more and 0.04% or less.
- B is a chemical element which improves hardenability.
- the B content is less than 0.0003%, there is insufficient effect of increasing hardenability.
- the B content is more than 0.0050%, such an effect becomes saturated and there is a deterioration in fatigue resistance due to intergranular fracture being more likely to occur as a result of B being precipitated at the grain boundaries.
- the B content be in a range of 0.0003% or more and 0.0050% or less, or more preferably in a range of 0.0010% or more and 0.0040% or less.
- Mo is a chemical element which improves hardenability
- Mo is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Mo content be 0.001% or more in order to realize such an effect. However, in the case where the Mo content is more than 2%, there is a significant deterioration in workability. In the case where Mo is added, it is preferable that the Mo content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- W is effective for increasing the strength of steel by forming carbides. It is preferable that the W content be 0.001% or more in order to realize such an effect. However, in the case where the W content is more than 2%, since unnecessary carbides are precipitated, there is a deterioration in fatigue resistance and there is a deterioration in workability. In the case where W is added, it is preferable that the W content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- Nb is a chemical element which improves hardenability and which contributes to an increase in strength by forming carbides. It is preferable that the Nb content be 0.001% or more in order to realize such effects. However, in the case where the Nb content is more than 0.1%, the effects become saturated and there is a deterioration in workability. In the case where Nb is added, it is preferable that the Nb content be 0.1% or less, or more preferably in a range of 0.001% or more and 0.04% or less.
- V is a chemical element which is effective for increasing the strength of steel by forming carbides and which has temper softening resistance. It is preferable that the V content be 0.001% or more in order to realize such effects. However, in the case where the V content is more than 0.1%, the effects become saturated and there is a deterioration in workability. In the case where V is added, it is preferable that the V content be 0.1% or less, or more preferably in a range of 0.001% or more and 0.5% or less
- Ni is a chemical element which improves hardenability
- Ni is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Ni content be 0.001% or more in order to realize such an effect. However, in the case where the Ni content is more than 2%, there is a significant deterioration in workability. In the case where Ni is added, it is preferable that the Ni content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- Cu is a chemical element which improves hardenability
- Cu is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Cu content be 0.001% or more in order to realize such an effect. However, in the case where the Cu content is more than 2%, there is a significant deterioration in workability. In the case where Cu is added, it is preferable that the Cu content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- Ca and REM are both chemical elements which are effective for suppressing the formation of the origins of cracks which induce a fatigue breaking in a use environment in which cyclic stress is applied by making the shape of non-metal inclusions spherical, these chemical elements may be selectively added as needed. Such an effect is recognized in the case where the content of each of Ca and REM is 0.0020% or more. On the other hand, in the case where the content is more than 0.02%, there is a decrease in cleaning level due to an increase in the amount of inclusions. Therefore, in the case where Ca or REM is added, it is preferable that the content of each of Ca and REM be 0.02% or less. In the case where Ca and REM are added in combination, it is preferable that the total content be 0.03% or less.
- the remainder of the chemical composition of the steel according to the present invention other than the constituents described above consists of Fe and inevitable impurities.
- the metallic microstructure according to the present invention is a microstructure in which the area ratio of pearlite is 85% or more and in which the total of the area ratios of ferrite and bainite (including 0) is 15% or less.
- the metallic microstructure include mainly pearlite and that the area ratio of pearlite be 85% or more to realize such an effect.
- the area ratio of pearlite is set to be 85% or more, and the total of the area ratios (including 0) of ferrite and bainite is set to be 15% or less.
- Prior austenite grain size 25 ⁇ m or more
- the strength of pearlite increases with decreasing lamellar spacing of pearlite.
- the lamellar spacing be 170 nm or less, or more preferably 150 nm or less.
- Hot-reduced steel pipes (having an outer diameter of 45 mm and a wall thickness of 4.5 mm) were manufactured, by performing hot rolling on steel slabs having steel chemical compositions (mass%) given in Table 1 in order to obtain hot-rolled steel strips, by performing roll forming and high-frequency resistance welding on the hot-rolled steel strips in order to manufacture electric resistance welded steel pipes (having an outer diameter of 89 mm and a wall thickness of 4.7 mm), and by thereafter performing hot reducing on the formed and welded pipes.
- product steel pipes were manufactured, by performing cold drawing in order to obtain cold drawn steel tubes (having an outer diameter of 40 mm and a thickness of 4.0 mm), and thereafter performing normalizing (at a temperature of 920°C for a duration of 10 minutes and with a cooling rate of 0.5°C/sec. to 3.0°C/sec. after soaking had been performed).
- tensile strength was determined.
- etching was performed so that austenite grain boundaries were exposed in a cross-section in the circumferential direction of the steel pipe in order to determine the austenite grain size.
- the grain size was determined based on a method of section by taking photographs of 10 microscopic fields using an optical microscope at a beautiful of 400 times, and the average value of the determined values was used as a representative value.
- a lamellar spacing of the pearlite was determined using a method of section, by performing a nital corrosion treatment on a cross-section in the circumferential direction of the steel pipe in the similar way as described above, and by taking photographs of 10 microscopic fields in which cementite layers were arranged as much at a right angle as possible to the paper plane using an electron scanning microscope of 20,000 times power, and the average value of the determined values was used as a representative value.
- the fatigue strength ⁇ w of the steel pipe was determined by performing a torsion fatigue test under conditions that the frequency was 3 Hz, the wave shape was a sine wave, and the stress ratio R was -1 (reversed vibration).
- ⁇ w was defined as the stress with which a fracture did not occur even after the number of the cycles reaches 2 million.
- the tensile strength was low in the case where the cooling rate for normalizing was in the lower range, and the torsion fatigue strength was low.
- the cooling rate was in the higher range, although the difference from the examples of the present invention in tensile strength was small, the torsion fatigue strength was lower than that of the examples of the present invention. The reason for that is thought to be because of the difference in the prior austenite grain size and because of the difference in the strength of pearlite.
- a hot-rolled steel sheet was used as a raw material of an electric resistance welded steel pipe in the present examples, the present invention is not limited to the examples, and a cold-rolled steel strip may be used as the raw material of a steel pipe.
- an ordinary electric resistance welded steel pipe which has not been subjected to hot reducing, may be used as a steel pipe which is subjected to cold drawing.
Abstract
Description
- The present invention relates to an electric resistance welded steel pipe excellent in terms of fatigue characteristic.
- In the automotive industry, in order to achieve weight saving and satisfactory stiffness property at the same time, there is a trend toward hollowing driving parts such as a drive shaft which have been manufactured using bar steel to date. As an example of raw materials used for such hollow parts, it is proposed to use a seamless steel pipe, and, for example, Patent Literature 1 discloses a hollow drive axis which is made from a seamless steel pipe as a raw material, having a steel chemical composition controlled to be within a specified range, which is excellent not only in terms of cold workability as indicated by an austenite grain size number of 9 or more after a quenching treatment has been performed but also in terms of hardenability, toughness, and torsion fatigue strength (hereinafter, also simply referred to as fatigue strength), and which realizes a stable fatigue life.
- However, in the case of a seamless steel pipe, there is a problem in that, since surface decarburization and surface defects tend to occur due to the method for manufacturing a seamless steel pipe, it is necessary to grind or polish the surface of the pipe in order to achieve satisfactory fatigue resistance, and in that, since a seamless pipe has unevenness and eccentricity in thickness, a seamless pipe is not always suitable for a rotated object.
- On the other hand, consideration has been given to using an electric resistance welded steel pipe for a drive shaft, because the problems described above are less likely to occur. For example, Patent Literature 2 discloses a technique for increasing the strength of a steel pipe by using an electric resistance welded steel pipe having a steel chemical composition controlled to be within a specified range as a raw material and by performing a quenching-tempering treatment on a weld of ERW and a portion around the weld as a hardening treatment.
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- [PTL 1] International Publication No.
WO2006/104023 - [PTL 2] Japanese Unexamined Patent Application
- However, although an electric resistance welded steel pipe is superior to a seamless steel pipe in terms of dimension accuracy, it is necessary to improve dimension accuracy by performing cold drawing in order to use an electric resistance welded steel pipe for applications such as a drive shaft for which very high dimension accuracy is required. In this case, it is necessary to perform normalizing after cold drawing has been performed. The reasons for that is because it is necessary to solve, for example, the following problems by performing normalizing: (1) a deterioration in toughness due to processing strain in the cold-drawn state, (2) a local increase in hardness in a weld of ERW due to a quenching effect caused by a thermal history in which rapid heating and rapid cooling occur when the welding is performed, and (3) a thin layer called a white layer, in which carbon concentration is low, in the bonded surface of a weld of ERW.
- In the case where normalizing is not performed, there is a risk in that, since an electric resistance welded steel pipe has low toughness, brittle failure may occur in a practical use environment. In addition, in the case of a drive shaft, since local stress concentration occurs in a weld of ERW and in the vicinity of the weld due to cyclic shearing stress and bending stress being applied, there is a risk in that fatigue breaking may occur in a short time. Therefore, normalizing is a treatment which is very important in order to use an electric resistance welded steel pipe for a drive shaft and which significantly influences the properties of a steel pipe as a final product.
- In the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe, the metallic microstructure widely varies from ferrite and pearlite to martensite due to a variation in cooling rate after normalizing has been performed. Therefore, since martensite microstructure may be formed, a tempering treatment becomes an indispensable process in order to achieve satisfactory toughness as disclosed in Patent Literature 1 and Patent Literature 2 in the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe, which results in a problem of an increase in manufacturing cost.
- An object of the present invention is, in order to solve the problems described above, to provide an electric resistance welded steel pipe whose metallic microstructure and tensile strength after normalizing has been performed are less likely to be influenced by a cooling rate when normalizing is performed even in the case where high-carbon steel is used as a raw material of an electric resistance welded steel pipe and with which stable fatigue strength can be achieved.
- The present inventors diligently conducted investigations in order to solve the problems described above, and as a result, found that, by controlling Al content in steel to be within an appropriate range, the metallic microstructure and tensile strength after normalizing has been performed become less likely to be influenced by a cooling rate after normalizing has been performed and that stable fatigue strength can be achieved. Moreover, it was found that, by controlling the prior austenite grain size to be within an appropriate range, it is possible to increase (1) the strength of pearlite and (2) fatigue crack propagation resistance of ferrite-pearlite steel without changing the tensile strength, which results in an increase in fatigue strength.
- The present inventors manufactured hot-reduced steel pipes (having an outer diameter of 45 mm and a wall thickness of 4.5 mm), by using a hot-rolled steel sheets (coiled at a coiling temperature of 650°C) having a basic chemical composition in accordance with the steel specification SAE1541 (containing 0.42%C, 1.5%Mn, 0.0035%N) and Al in various amounts as a raw material, by performing roll forming and high-frequency resistance welding on the raw material in order to manufacture electric resistance welded steel pipes (having an outer diameter of 89 mm and a wall thickness of 4.7 mm), and by thereafter performing hot reducing on the formed and welded pipes. Subsequently, by performing cold drawing in order to make cold-drawn pipes (having an outer diameter of 40 mm and a wall thickness of 4.0 mm), and by thereafter performing normalizing (at a temperature of 920°C for a holding time of 10 minutes and with a cooling rate of 0.5°C/sec. to 3.0°C/sec. after soaking had been performed), product steel pipes were manufactured.
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Fig. 1 illustrates the relationship between a cooling rate for normalizing and HV hardness (Vickers hardness). It is clarified that, in the case where the Al content is 0.005% or less, almost constant HV hardness is achieved for a wide cooling rate range, that, in the case where the Al content is 0.007% or more, HV hardness is strongly influenced by the cooling rate, and that, in the case where the cooling rate is low, there is a sharp decrease in HV hardness. -
Fig. 2 illustrates the relationship between the Al content and a lamellar spacing,Fig. 3 illustrates the relationship between the Al content and a prior austenite grain size, andFig. 4 illustrates the relationship between the Al content and torsion fatigue strength. Here, the cooling rate for normalizing was 1°C/sec. The prior austenite grain size increases with decreasing Al content, and the torsion fatigue strength increases along with the prior austenite grain size. It is clarified that, in the case where the Al content is 0.005% or less, such an effect becomes saturated and that the torsion fatigue strength also becomes stable. -
Fig. 5 illustrates the results of the cross-section observation of the fracture portion after a fatigue test had been performed, andFig. 5(a) and Fig. 5 (b) respectively illustrate the fatigue crack propagation situations for a material containing 0.03%-Al and a material containing 0.003%-Al. The crack propagation route is indicated with a white line. It was found that fatigue crack starts from the outer surface side of a pipe and then propagates through a winding path made of soft pro-eutectoid ferrite. In addition, it is presumed that, since the crack meanders in a zig-zag manner, and since the degree of a change in direction increases with increasing apparent pearlite grain size (corresponding to a prior austenite grain), which is surrounded by the pro-eutectoid ferrite, there is an improvement in crack propagation resistance, which results in an increase in fatigue strength. - The reason why the results illustrated in
Fig. 1, Fig. 2 , andFig. 3 were obtained is thought to be as follows. That is, since the amount of aluminum nitride, which has been precipitated before normalizing is performed, decreases with decreasing Al content, there is a decrease in the pinning effect of aluminum nitride, which results in a tendency for an austenite grain size to increase in a normalizing process. Since pearlite and ferrite use prior austenite grain boundaries as their transformation sites, in the case where there is a decrease in grain boundary area due to an increase in prior austenite grain size, there is a decrease in the number of transformation sites, which results in a decrease in the fraction of ferrite. In particular, the reason why hardness varies depending on the Al content in a low cooling rate region inFig. 1 is because, in the case where the Al content is high, since the growth of austenite grains is suppressed in a normalizing process due to the pinning effect of aluminum nitride (AlN) which has been precipitated before normalizing is performed, and, at the same time, since there is an increase in the lamellar spacing of pearlite which is finally formed, there is a decrease in hardness. A decrease in hardness is significant particularly in a low cooling rate region, in which quenching effect is less likely to be realized, and significantly depends on the Al content (the amount of AlN precipitated) in steel. In the case where the Al content is 0.005% or less, since there is a decrease in the amount of aluminum nitride (AlN) precipitated, and since aluminum nitride is dissolved in a normalizing process even if aluminum nitride is precipitated in advance, there is a decrease in pinning effect, which results in a decrease in the lamellar spacing of pearlite due to austenite grains growing easily, and which results in a decrease in a change in hardness depending on a cooling rate. - The relationships of an austenite grain size to a lamellar spacing and strength are thought to be as follows. That is, in the case where an austenite grain size is large, since there is a decrease in the number of pearlite transformation sites (mainly austenite grain boundaries), there is a decrease in pearlite transformation temperature. As a result, it is considered that, since there is an increase in the temperature difference between the pearlite equilibrium transformation temperature and the transformation starting temperature, that is, the degree of undercooling, there is a decrease in lamellar spacing, which results in an increase in the strength of pearlite as expected based on the conventionally-known relationship between a lamellar spacing and the strength of pearlite. As a result, it is considered that, since a fatigue crack becomes less likely to penetrate pearlite microstructure due to an increase in the strength of pearlite, the crack propagates in a zig-zag manner avoiding the pearlite, which results in an increase in fatigue strength due to an increase in fatigue crack propagation resistance.
- The present invention has been completed on the basis of the knowledge described above and further investigations, and the subject matter of the present invention is as follows.
- [1] An electric resistance welded steel pipe, the steel pipe having a chemical composition containing, by mass%, C: 0.35% or more and 0.55% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005% or less, N: 0.0050% or less, Cr: 0.1% or more and 0.5% or less, and the balance being Fe and inevitable impurities and a metallic microstructure including pearlite, ferrite, and bainite, in which the area ratio of the pearlite is 85% or more, the total of the area ratios (including 0) of the ferrite and the bainite is 15% or less, and in which a prior austenite grain size is 25 µm or more.
- [2] The electric resistance welded steel pipe excellent in terms of fatigue characteristic according to item [1], the steel pipe having the chemical composition further containing, by mass%, one or more selected from among Ti: 0.005% or more and 0.1% or less, B: 0.0003% or more and 0.0050% or less, Mo: 2% or less, W: 2% or less, Nb: 0.1% or less, V: 0.1% or less, Ni: 2% or less, Cu: 2% or less, Ca: 0.02% or less, and REM: 0.02% or less.
- According to the present invention, it is possible to obtain an electric resistance welded steel pipe having satisfactory fatigue resistance which is required for a drive shaft.
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- [
Fig. 1] Fig. 1 is a diagram illustrating a relationship between a cooling rate when normalizing is performed and HV hardness. - [
Fig. 2] Fig. 2 is a diagram illustrating a relationship between Al content in steel and lamellar spacing. - [
Fig. 3] Fig. 3 is a diagram illustrating a relationship between Al content in steel and a prior austenite grain size. - [
Fig. 4] Fig. 4 is a diagram illustrating a relationship between Al content in steel and torsion fatigue strength. - [
Fig. 5] Fig. 5 is a diagram illustrating the propagation behavior of a fatigue crack. ((a)material containing 0.03%-Al, and (b) material containing 0.003%-Al) - The reasons for the limitations on the constituent features of the present invention will be described hereafter.
- First, the reasons for the limitations on the chemical composition of the steel according to the present invention will be described. Here, % used when describing a chemical composition always represents mass%.
- In the case where the C content is less than 0.35%, it is not possible to achieve satisfactory strength or desired fatigue resistance. On the other hand, in the case where the C content is more than 0.55%, since there is a deterioration in weldability, it is not possible to achieve a stable welding quality of ERW. Therefore, the C content is set to be in a range of 0.35% or more and 0.55% or less, or preferably in a range of 0.40% or more and 0.45% or less.
- There is a case where Si is added for deoxidation, and it is not possible to realize a sufficient deoxidation effect in the case where the Si content is less than 0.01%. At the same time, Si is also a solute strengthening element, and it is necessary that the Si content be 0.01% or more in order to realize such an effect. On the other hand, in the case where the Si content is more than 1.0%, there is a deterioration in the hardenability of a steel pipe. The Si content is set to be in a range of 0.01° or more and 1.0% or less, or preferably 0.1% or more and 0.4% or less.
- Mn is a chemical element which promotes pearlite transformation and improves hardenability, it is necessary that the Mn content be 1.0% or more in order to realize such effects. On the other hand, in the case where the Mn content is more than 3.0%, there is a deterioration in the welding quality of ERW, and in addition, there is a deterioration in fatigue resistance due to an increase in the amount of residual austenite. The Mn content is set to be in a range of 1.0% or more and 3.0% or less, or preferably in a range of 1.4% or more and 2.0% or less.
- P is an inevitable impurity in the present invention, and the upper limit of the P content is set to be 0.02% or less. There is a tendency for P to be concentrated in a segregation part which is formed when continuous casting is performed and to remain in a hot-rolled steel sheet as a raw material of a pipe. Since the edges of a steel strip are abutted and subjected to upsetting when electric resistance welding is performed, the segregation part in which P is concentrated may be exposed on the outer surface and inner surface of a steel pipe, which results in there being a risk in that cracking occurs when secondary processing such as flattening forming is performed on this part. Therefore, it is preferable that the P content be 0.01% or less.
- S is an inevitable impurity in the present invention, and the upper limit of the S content is set to be 0.01% or less. In the case where the S content is high, there is a deterioration in toughness of raw material, and S combines with Mn in steel to form MnS. Since MnS is elongated in the longitudinal direction of a steel sheet to form a long inclusion in a hot rolling process, there is a deterioration in workability and toughness. Therefore, it is preferable that the S content be 0.005% or less, or more preferably 0.003% or less.
- Although Al is an important chemical element in the present invention in order to achieve the desired prior austenite grain size accompanied by satisfactory torsion fatigue strength, since, in the case where the Al content is more than 0.005%, a pinning effect is realized in a normalizing process due to an increase in the amount of AlN precipitated, which results in the desired austenite grain size not being achieved due to the growth of austenite grains being suppressed. Therefore, the Al content is set to be 0.005% or less, or preferably 0.003% or less.
- Since N is a chemical element which contributes to suppressing the growth of austenite grains in a normalizing process as a result of combining with Al to form AlN, it is necessary that the N content be 0.0050% or less in order to suppress such an effect, or preferably 0.0035% or less.
- Since Cr is a chemical element which decreases the pearlite transformation temperature, there is a decrease in the lamellar spacing of pearlite, which results in an increase in torsion fatigue strength due to an increase in the strength of pearlite. It is necessary that the Cr content be 0.1% or more in order to realize such an effect. On the other hand, in the case where the Cr content is more than 0.5%, since Cr forms oxides, and since the oxides may remain in a weld of ERW, there may be a deterioration in weldability of ERW. Therefore, the Cr content is set to be in a range of 0.1% or more and 0.5% or less, or preferably in a range of 0.15% or more and 0.30% or less.
- The basic chemical composition according to the present invention is as described above, and one or more of Ti, B, Mo, W, Nb, V, Ni, Cu, Ca, and REM, which will be described below, may further be added in order to increase strength and fatigue strength.
- Ti is effective for fixing N in steel in the form of TiN. However, in the case where the Ti content is less than 0.005%, there is insufficient effect of fixing N, and, in the case where the Ti content is more than 0.1%, there is a deterioration in the workability and toughness of steel. In the case where Ti is added, it is preferable that the Ti content be in a range of 0.005% or more and 0.1% or less, or more preferably in a range of 0.01% or more and 0.04% or less.
- B is a chemical element which improves hardenability. In the case where the B content is less than 0.0003%, there is insufficient effect of increasing hardenability. On the other hand, in the case where the B content is more than 0.0050%, such an effect becomes saturated and there is a deterioration in fatigue resistance due to intergranular fracture being more likely to occur as a result of B being precipitated at the grain boundaries. In the case where B is added, it is preferable that the B content be in a range of 0.0003% or more and 0.0050% or less, or more preferably in a range of 0.0010% or more and 0.0040% or less.
- Since Mo is a chemical element which improves hardenability, Mo is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Mo content be 0.001% or more in order to realize such an effect. However, in the case where the Mo content is more than 2%, there is a significant deterioration in workability. In the case where Mo is added, it is preferable that the Mo content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- W is effective for increasing the strength of steel by forming carbides. It is preferable that the W content be 0.001% or more in order to realize such an effect. However, in the case where the W content is more than 2%, since unnecessary carbides are precipitated, there is a deterioration in fatigue resistance and there is a deterioration in workability. In the case where W is added, it is preferable that the W content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- Nb is a chemical element which improves hardenability and which contributes to an increase in strength by forming carbides. It is preferable that the Nb content be 0.001% or more in order to realize such effects. However, in the case where the Nb content is more than 0.1%, the effects become saturated and there is a deterioration in workability. In the case where Nb is added, it is preferable that the Nb content be 0.1% or less, or more preferably in a range of 0.001% or more and 0.04% or less.
- V is a chemical element which is effective for increasing the strength of steel by forming carbides and which has temper softening resistance. It is preferable that the V content be 0.001% or more in order to realize such effects. However, in the case where the V content is more than 0.1%, the effects become saturated and there is a deterioration in workability. In the case where V is added, it is preferable that the V content be 0.1% or less, or more preferably in a range of 0.001% or more and 0.5% or less
- Since Ni is a chemical element which improves hardenability, Ni is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Ni content be 0.001% or more in order to realize such an effect. However, in the case where the Ni content is more than 2%, there is a significant deterioration in workability. In the case where Ni is added, it is preferable that the Ni content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- Since Cu is a chemical element which improves hardenability, Cu is effective for increasing fatigue strength by increasing the strength of steel. It is preferable that the Cu content be 0.001% or more in order to realize such an effect. However, in the case where the Cu content is more than 2%, there is a significant deterioration in workability. In the case where Cu is added, it is preferable that the Cu content be 2% or less, or more preferably in a range of 0.001% or more and 0.5% or less.
- Since Ca and REM are both chemical elements which are effective for suppressing the formation of the origins of cracks which induce a fatigue breaking in a use environment in which cyclic stress is applied by making the shape of non-metal inclusions spherical, these chemical elements may be selectively added as needed. Such an effect is recognized in the case where the content of each of Ca and REM is 0.0020% or more. On the other hand, in the case where the content is more than 0.02%, there is a decrease in cleaning level due to an increase in the amount of inclusions. Therefore, in the case where Ca or REM is added, it is preferable that the content of each of Ca and REM be 0.02% or less. In the case where Ca and REM are added in combination, it is preferable that the total content be 0.03% or less.
- The remainder of the chemical composition of the steel according to the present invention other than the constituents described above consists of Fe and inevitable impurities.
- The metallic microstructure according to the present invention is a microstructure in which the area ratio of pearlite is 85% or more and in which the total of the area ratios of ferrite and bainite (including 0) is 15% or less.
- In order to increase fatigue strength by increasing fatigue crack propagation resistance as a result of a fatigue crack propagating in a zig-zag manner as described above, it is necessary that the metallic microstructure include mainly pearlite and that the area ratio of pearlite be 85% or more to realize such an effect. On the other hand, in the case where the total of the area ratios (including 0) of soft ferrite and bainite, which is hard but not so effective than pearlite, is more than 15%, there is a decrease in the effect of increasing fatigue strength. Therefore, the area ratio of pearlite is set to be 85% or more, and the total of the area ratios (including 0) of ferrite and bainite is set to be 15% or less.
- Since the degree of the deflection of a fatigue crack increases with increasing apparent grain size of pearlite which is surrounded by ferrite layers, there is an improvement in crack propagation resistance. Since ferrite is formed at prior austenite grain boundaries, the apparent pearlite grain size increases with increasing prior austenite grain size. It is necessary that the prior austenite grain size be 25 µm or more in order to improve crack propagation resistance, and there is an insufficient improvement in fatigue crack propagation resistance in the case where the prior austenite grain size is less than 25 µm.
- It is conventionally known that the strength of pearlite increases with decreasing lamellar spacing of pearlite. In order to increase the strength of pearlite so that a fatigue crack does not penetrate the pearlite and goes around the pearlite, it is preferable that the lamellar spacing be 170 nm or less, or more preferably 150 nm or less.
- Hot-reduced steel pipes (having an outer diameter of 45 mm and a wall thickness of 4.5 mm) were manufactured, by performing hot rolling on steel slabs having steel chemical compositions (mass%) given in Table 1 in order to obtain hot-rolled steel strips, by performing roll forming and high-frequency resistance welding on the hot-rolled steel strips in order to manufacture electric resistance welded steel pipes (having an outer diameter of 89 mm and a wall thickness of 4.7 mm), and by thereafter performing hot reducing on the formed and welded pipes. Subsequently, product steel pipes were manufactured, by performing cold drawing in order to obtain cold drawn steel tubes (having an outer diameter of 40 mm and a thickness of 4.0 mm), and thereafter performing normalizing (at a temperature of 920°C for a duration of 10 minutes and with a cooling rate of 0.5°C/sec. to 3.0°C/sec. after soaking had been performed).
- Using a tensile specimen (JIS No. 12 specimen) which had been collected from the product steel pipe so that the longitudinal direction is the axis direction of the steel pipe, tensile strength was determined. In addition, etching was performed so that austenite grain boundaries were exposed in a cross-section in the circumferential direction of the steel pipe in order to determine the austenite grain size. The grain size was determined based on a method of section by taking photographs of 10 microscopic fields using an optical microscope at a magnificent of 400 times, and the average value of the determined values was used as a representative value.
- In addition, a lamellar spacing of the pearlite was determined using a method of section, by performing a nital corrosion treatment on a cross-section in the circumferential direction of the steel pipe in the similar way as described above, and by taking photographs of 10 microscopic fields in which cementite layers were arranged as much at a right angle as possible to the paper plane using an electron scanning microscope of 20,000 times power, and the average value of the determined values was used as a representative value.
- The fatigue strength σw of the steel pipe was determined by performing a torsion fatigue test under conditions that the frequency was 3 Hz, the wave shape was a sine wave, and the stress ratio R was -1 (reversed vibration). Here, σw was defined as the stress with which a fracture did not occur even after the number of the cycles reaches 2 million. These evaluation results of the properties are given in Table 2 and Table 3.
[Table 2] Pipe No. Steel Grade Kind of Pipe Reducing Condition Cold Drawing Area Reduction (%) Normalizing Condition Area Fraction of Microstructure % Prior Austenite Grain Size (nm) Lamellar Spacing (nm) Tensile Strength TS (MPa) Torsion Fatigue Strength σw (MPa) Strength TS Stability Result From Cooling Rate Note Heating Temperature (°C) Finish Rolling Temperature (°C) Reducing Ratio (%) Soaking Condition Cooling Rate (°C/sec.) Pearlite Ferrite Bainite 1 A Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 32 164 835 175 ○ Example 2 1.0 96.0 4.0 0.0 31 162 840 175 Example 3 2.0 93.0 2.0 5.0 35 161 845 180 Example 4 3.0 92.0 1.0 7.0 33 159 850 180 Example 5 B Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 94.0 6.0 0.0 34 165 832 175 ○ Example 6 1.0 96.0 4.0 0.0 32 165 833 175 Example 7 2.0 92.0 3.0 5.0 35 162 840 175 Example 8 3.0 90.0 1.0 9.0 36 158 852 180 Example 9 C Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 32 162 840 175 ○ Example 10 1.0 95.0 4.0 1.0 34 164 835 175 Example 11 2.0 93.0 2.0 5.0 33 167 825 175 Example 12 3.0 92.0 1.0 7.0 31 162 840 175 Example 13 D Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 31 164 835 175 ○ Example 14 1.0 96.0 4.0 0.0 29 159 850 180 Example 15 2.0 91.0 2.0 7.0 30 164 836 175 Example 16 3.0 92.0 1.0 7.0 33 163 838 175 Example 17 E Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 35 162 840 175 ○ Example 18 1.0 96.0 4.0 0.0 34 161 845 180 Example 19 2.0 92.0 2.0 6.0 35 159 850 180 Example 20 3.0 90.0 1.0 9.0 33 163 839 175 Example 21 F Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 32 166 830 175 ○ Example 22 1.0 96.0 4.0 0.0 29 162 840 175 Example 23 2.0 93.0 2.0 5.0 33 161 845 180 Example 24 3.0 91.0 1.0 8.0 36 159 850 180 Example 25 G Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 35 162 840 175 ○ Example 26 1.0 96.0 4.0 0.0 36 161 845 180 Example 27 2.0 93.0 3.0 4.0 35 163 839 175 Example 28 3.0 90.0 1.0 9.0 32 161 843 180 Example 29 H Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 95.0 5.0 0.0 33 162 840 175 ○ Example 30 1.0 96.0 4.0 0.0 36 164 835 175 Example 31 2.0 93.0 3.0 4.0 35 159 850 180 Example 32 3.0 91.0 2.0 7.0 33 157 855 180 Example [Table 3] Pipe No. Steel Grade Kind of Pipe Reducing Condition Cold Drawing Area Reduction (%) Normalizing Condition Area Fraction of Microstructure % Prior Austenite Grain Size (nm) Lamellar Spacing (nm) Tensile Strength TS (MPa) Torsion Fatigue Strength σw (MPa) Strength TS Stability Result From Cooling Rate Note Heating Temperature (°C) Finish Rolling Temperature (°C) Reducing Ratio (%) Soaking Condition Cooling Rate (°C/sec.) Pearlite Ferrite Bainite 33 I Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 96.0 4.0 0.0 32 162 842 175 ○ Example 34 1.0 94.0 4.0 2.0 33 164 836 175 Example 35 2.0 91.0 3.0 6.0 34 162 840 175 Example 36 3.0 89.0 2.0 9.0 33 159 850 180 Example 37 J Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 96.0 4.0 0.0 36 162 840 175 ○ Example 38 1.0 96.0 3.0 1.0 35 161 845 180 Example 39 2.0 92.0 3.0 5.0 33 166 830 175 Example 40 3.0 90.0 2.0 8.0 36 160 846 180 Example 41 K Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 75.0 25.0 0.0 19 196 752 140 × Comparative Example 42 1.0 78.0 22.0 0.0 20 185 780 145 Comparative Example 43 2.0 80.0 20.0 0.0 21 177 800 145 Comparative Example 44 3.0 82.0 18.0 0.0 23 172 812 150 Comparative Example 45 L Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 75.0 25.0 0.0 18 191 765 140 × Comparative Example 46 1.0 76.0 24.0 0.0 20 185 780 140 Comparative Example 47 2.0 80.0 20.0 0.0 22 176 802 150 Comparative Example 48 3.0 81.0 19.0 0.0 22 169 820 150 Comparative Example 49 M Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 80.0 20.0 0.0 18 197 750 135 × Comparative Example 50 1.0 84.0 16.0 0.0 17 187 775 140 Comparative Example 51 2.0 82.0 17.0 1.0 20 177 800 150 Comparative Example 52 3.0 80.0 16.0 4.0 21 173 810 150 Comparative Example 53 N Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 79.0 21.0 0.0 17 200 745 135 × Comparative Example 54 1.0 80.0 20.0 0.0 18 189 768 140 Comparative Example 55 2.0 81.0 17.0 2.0 19 175 805 150 Comparative Example 56 3.0 81.0 16.0 3.0 22 171 815 150 Comparative Example 57 O Reduced Steel Pipe 950 830 49 21 920°C ×10 minutes 0.5 78.0 22.0 0.0 21 202 740 135 × Comparative Example 58 1.0 78.0 22.0 0.0 22 185 780 145 Comparative Example 59 2.0 79.0 14.0 7.0 21 173 810 150 Comparative Example 60 3.0 76.0 16.0 8.0 23 171 815 150 Comparative Example Annotation: An underlined evaluated value indicates a value out of the range according to the present invention. - Here, regarding strength stability, a case where the deviation (the difference between the maximum value and the minimum value) of tensile strength TS when the cooling rate for normalizing was changed in the range of 0.5°C/sec. to 3.0°C/sec. was 50 MPa or less was judged as satisfactory (○), and a case where the deviation was more than 50 MPa was judged as unsatisfactory (x).
- As Table 2 and Table 3 indicate, it is clarified that the electric resistance welded steel pipes according to the present invention were all excellent in terms of strength stability as indicated by the small deviation of strength caused by the change in the cooling rate for normalizing, had high fatigue crack propagation resistance as indicated by the strength stability, the small lamellar spacing, and the large prior austenite grain size, and stably had high torsion fatigue strength.
- On the other hand, in the case of a raw material having a high Al content of more than the range according to the present invention, the tensile strength was low in the case where the cooling rate for normalizing was in the lower range, and the torsion fatigue strength was low. In addition, in the case where the cooling rate was in the higher range, although the difference from the examples of the present invention in tensile strength was small, the torsion fatigue strength was lower than that of the examples of the present invention. The reason for that is thought to be because of the difference in the prior austenite grain size and because of the difference in the strength of pearlite.
- Here, although a hot-rolled steel sheet was used as a raw material of an electric resistance welded steel pipe in the present examples, the present invention is not limited to the examples, and a cold-rolled steel strip may be used as the raw material of a steel pipe. Also, an ordinary electric resistance welded steel pipe, which has not been subjected to hot reducing, may be used as a steel pipe which is subjected to cold drawing.
Claims (2)
- An electric resistance welded steel pipe, the steel pipe having a chemical composition containing, by mass%, C: 0.35% or more and 0.55% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005% or less, N: 0.0050% or less, Cr: 0.1% or more and 0.5% or less, and the balance being Fe and inevitable impurities and a metallic microstructure including pearlite, ferrite, and bainite, wherein the area ratio of the pearlite is 85% or more, the total of the area ratios (including 0) of the ferrite and the bainite is 15% or less, and wherein a prior austenite grain size is 25 µm or more.
- The electric resistance welded steel pipe according to Claim 1, the steel pipe having the chemical composition further containing, by mass%, one or more selected from among Ti: 0.005% or more and 0.1% or less, B: 0.0003% or more and 0.0050% or less, Mo: 2% or less, W: 2% or less, Nb: 0.1% or less, V: 0.1% or less, Ni: 2% or less, Cu: 2% or less, Ca: 0.02% or less, and REM: 0.02% or less.
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JP2013016670 | 2013-01-31 | ||
PCT/JP2014/052708 WO2014119802A1 (en) | 2013-01-31 | 2014-01-30 | Electric-resistance-welded steel pipe |
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EP2952601A4 EP2952601A4 (en) | 2016-02-17 |
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EP (1) | EP2952601B1 (en) |
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2014
- 2014-01-30 JP JP2014559807A patent/JP5892267B2/en active Active
- 2014-01-30 EP EP14746008.3A patent/EP2952601B1/en active Active
- 2014-01-30 WO PCT/JP2014/052708 patent/WO2014119802A1/en active Application Filing
- 2014-01-30 US US14/765,206 patent/US20150368768A1/en not_active Abandoned
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Cited By (4)
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EP3395973A4 (en) * | 2015-12-21 | 2019-06-12 | Nippon Steel & Sumitomo Metal Corporation | As-rolled type k55 electric-resistance-welded oil well pipe, and hot-rolled steel plate |
US10738371B2 (en) | 2015-12-21 | 2020-08-11 | Nippon Steel Corporation | As-rolled type K55 electric resistance welded oil well pipe and hot-rolled steel sheet |
EP3733911A4 (en) * | 2017-12-26 | 2020-11-25 | Posco | Ultra-high-strength hot-rolled steel sheet, steel pipe, member, and manufacturing methods therefor |
US11939639B2 (en) | 2017-12-26 | 2024-03-26 | Posco Co., Ltd | Ultra-high-strength hot-rolled steel sheet, steel pipe, member, and manufacturing methods therefor |
Also Published As
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KR20150099831A (en) | 2015-09-01 |
KR101710816B1 (en) | 2017-02-27 |
CN104968821B (en) | 2017-03-08 |
WO2014119802A1 (en) | 2014-08-07 |
JPWO2014119802A1 (en) | 2017-01-26 |
CN104968821A (en) | 2015-10-07 |
EP2952601B1 (en) | 2017-09-27 |
EP2952601A4 (en) | 2016-02-17 |
US20150368768A1 (en) | 2015-12-24 |
JP5892267B2 (en) | 2016-03-23 |
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