WO2014119802A1 - 電縫鋼管 - Google Patents
電縫鋼管 Download PDFInfo
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- WO2014119802A1 WO2014119802A1 PCT/JP2014/052708 JP2014052708W WO2014119802A1 WO 2014119802 A1 WO2014119802 A1 WO 2014119802A1 JP 2014052708 W JP2014052708 W JP 2014052708W WO 2014119802 A1 WO2014119802 A1 WO 2014119802A1
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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
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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
- 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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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
- C22C38/002—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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- 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
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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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- 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
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- 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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- 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
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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
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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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
- C22C38/32—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
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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
- C22C38/42—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 having excellent fatigue characteristics.
- Patent Document 1 discloses a seamless steel pipe whose steel composition is controlled within a desired range. Excellent cold workability, hardenability, toughness and torsion fatigue strength with austenite grain size number of 9 or more as a raw material after quenching ) (Hereinafter simply referred to as fatigue strength) and a hollow drive shaft that exhibits a stable fatigue life is disclosed.
- the seamless steel pipe has a large surface decarburization and surface flaws due to its manufacturing method.
- the surface In order to obtain sufficient fatigue resistance, the surface must be ground and polished.
- meat unevenness and sensibility in thickness
- Patent Document 2 an electric resistance steel pipe whose steel composition is controlled within a desired range is used as a raw material, and a hardening treatment is performed by quenching and tempering the electric seam portion (weld of ERW) and the vicinity thereof. And the technique which raises the intensity
- ERW steel pipes have superior dimensional accuracy compared to seamless steel pipes, dimensional accuracy by cold drawing for dimensions such as drive shafts that require very high dimensional accuracy (dimension accuracy). ) Needs to be improved. In that case, it is necessary to normalize after cold drawing. The reason for this is (1) toughness is reduced due to the effect of processing strain as it is cold drawn, and (2) rapid welding during welding and thermal history of quenching (thermal) (History) has been hardened locally, (3) There is a thin layer called white layer with a low carbon concentration on the joint surface of ERW welding, etc. This is to eliminate the problem.
- the ERW steel pipe has a low toughness, so there is a risk of brittle failure in the actual use environment.
- a drive shaft since repeated shearing stress and bending stress are applied, local stress concentration occurs in the ERW welded part and the vicinity thereof, resulting in a short time. There is a risk of causing fatigue breaking at the end of life. Therefore, the normalizing process is extremely important in applying the electric resistance welded steel pipe to the drive shaft, and at the same time has a great influence on the characteristics of the steel pipe as the final product.
- An object of the present invention is to provide an electric resistance steel pipe capable of ensuring fatigue strength.
- the inventors have controlled the Al content in the steel within an appropriate range, so that the metal structure and tensile strength after normalization are cooled after normalization. Stable fatigue strength can be secured without being affected by speed. Furthermore, by controlling the prior austenite grain size (primary austenite grain size) within an appropriate range, even if it is ferrite or pearlite steel having the same tensile strength, (1) the strength of pearlite itself is high, (2) It has been found that fatigue crack propagation resistance can be increased, and higher fatigue strength can be obtained.
- the inventors use steel standard SAE1541 (0.42% C-1.5% Mn-0.0035% N) as a basic component and hot rolled steel sheet (coiling temperature 650 ° C.) with varying Al content as material. Then, this was made into an electric-welded steel pipe (outer diameter 89 mm, wall thickness 4.7 mm) by roll forming and high-frequency resistance welding, and then reduced-reduced steel pipe (hot reducing) by hot reduction rolling (hot reduction) ( The outer diameter was 45 mm and the wall thickness was 4.5 mm. Then, after cold-drawn steel pipe (outer diameter 40 mm, wall thickness 4.0 mm) by cold drawing, normalization (920 ° C. ⁇ 10 minutes hold, cooling rate after soaking 0.5-3.0 ° C. / s) to obtain a product steel pipe.
- FIG. 1 shows the relationship between the cooling rate of normalization and the HV hardness (Vickers hardness).
- the Al content is 0.005% or less, an almost constant HV hardness can be obtained over a wide range of cooling rates, whereas when the Al amount is 0.007% or more, the HV hardness has an influence on the cooling rate. It can be seen that the HV hardness is drastically reduced when the cooling rate is low.
- FIG. 2 shows the relationship between Al content and lamella spacing
- FIG. 3 shows the relationship between Al content and prior austenite grain size
- FIG. 4 shows the relationship between Al content and torsional fatigue strength.
- the normalization cooling rate was 1 ° C./s.
- the Al content decreases, the prior austenite grains become coarser, and the torsional fatigue strength increases accordingly. It can be seen that when the Al content is 0.005% or less, the effect is saturated and the torsional fatigue strength is stabilized.
- FIG. 5 shows the result of cross-section observation of the fractured portion after the fatigue test, where FIG. 5 (a) is a 0.03% Al material, and FIG. 5 (b) is 0. It is a figure which shows the fatigue crack propagation condition in 0.003% Al material. The propagation path of the crack (propagation route) is indicated by a white line. It was found that fatigue cracks originate from the outer surface side of the tube, and then propagate as cracks are sewn with soft pro-eutectoid ferrite.
- the austenite grains in the normalizing process were caused by the pinning effect caused by aluminum nitride (AlN) deposited before normalization.
- AlN aluminum nitride
- the lamella spacing of the finally produced pearlite is widened, so the hardness is lowered.
- the amount of decrease in hardness is particularly remarkable in a low cooling rate region where baking is difficult to occur, and strongly depends on the amount of Al in steel (the amount of precipitated AlN).
- AlN aluminum nitride
- the relationship between the austenite grain size and the lamellar spacing and strength is considered as follows. That is, when the austenite grain size is large, the pearlite transformation sites (mainly austenite grain boundaries) are reduced, so that the pearlite transformation temperature is lowered. As a result, the temperature difference from the pearlite equilibria transformation temperature to the transformation start point, that is, the degree of supercooling (degree of undercooling) increases, so that the lamella interval is narrowed and the conventionally known lamella interval. It is considered that the intensity of pearlite increases according to the relationship between the intensity of pearlite and pearlite.
- the increase in pearlite strength makes it difficult for fatigue cracks to penetrate the pearlite structure, and the cracks propagate through the pearlite in a zigzag manner, improving fatigue crack propagation resistance and leading to an increase in fatigue strength. Conceivable.
- the present invention has been made by further studying the above-described knowledge, and the gist thereof is as follows.
- Component composition is mass%, C: 0.35-0.55%, Si: 0.01-1.0%, Mn: 1.0-3.0%, P: 0.02%
- S 0.01% or less
- Al 0.005% or less
- N 0.0050% or less
- Cr 0.1 to 0.5%
- the structure is composed of pearlite, ferrite and bainite, the area fraction of the pearlite is 85% or more, the sum of the area fraction of the ferrite and the area fraction of the bainite (including 0) is 15% or less, and the former austenite ERW steel pipe having a particle size of 25 ⁇ m or more.
- an electric resistance welded steel pipe having fatigue resistance necessary as a drive shaft can be obtained.
- FIG. 1 is a diagram for explaining the relationship between the cooling rate in normalization and the HV hardness.
- FIG. 2 is a diagram showing the relationship between the amount of Al in steel and the lamella spacing.
- FIG. 3 is a graph showing the relationship between the amount of Al in steel and the prior austenite grain size.
- FIG. 4 is a diagram showing the relationship between the amount of Al in steel and torsional fatigue strength.
- FIG. 5 is a diagram for explaining the propagation behavior of fatigue cracks. ((A) 0.03% Al material, (b) 0.003% Al material)
- C 0.35-0.55% If C is less than 0.35%, sufficient strength cannot be obtained, and required fatigue resistance characteristics cannot be obtained. On the other hand, if it exceeds 0.55%, the weldability is deteriorated, so that stable ERW welding quality cannot be obtained. Therefore, the C content is in the range of 0.35 to 0.55%. Preferably it is 0.40 to 0.45% of range.
- Si 0.01 to 1.0% Si may be added for deoxidation, and if it is less than 0.01%, a sufficient deoxidation effect cannot be obtained. At the same time, Si is also a solid solution strengthening element, and in order to obtain the effect, it is necessary to contain 0.01% or more. On the other hand, if it exceeds 1.0%, the hardenability of the steel pipe decreases.
- the Si amount is in the range of 0.01 to 1.0%. Preferably it is 0.1 to 0.4%.
- Mn 1.0 to 3.0%
- Mn is an element that promotes pearlite transformation and improves hardenability, and 1.0% or more must be added to obtain the effect. On the other hand, if it exceeds 3.0%, the welding quality of ERW is lowered, the amount of retained austenite is increased, and the fatigue resistance is lowered.
- the Mn content is in the range of 1.0 to 3.0%. Preferably it is 1.4 to 2.0% of range.
- P 0.02% or less
- P is an unavoidable impurity, and the upper limit of the amount is 0.02% or less.
- P tends to concentrate in a segregation part formed during continuous casting, and remains even in a hot-rolled steel sheet made of a tube material. Since the edge of the steel strip is butt-upset (upset) during ERW welding, the segregated portion where P is concentrated may be exposed on the outer surface and inner surface of the tube, and flattening (flattening) is applied to this portion. There is a risk of cracking when secondary processing such as forming is applied. Therefore, it is preferably 0.01% or less.
- S 0.01% or less
- S is an unavoidable impurity, and its upper limit is made 0.01% or less.
- the amount of S is large, it reduces the toughness of the material and also combines with Mn in the steel to form MnS. This becomes a long inclusion stretched in the longitudinal direction in the hot rolling process, and deteriorates workability and toughness. Therefore, it is preferably 0.005% or less, more preferably 0.003% or less.
- Al 0.005% or less
- Al is an important element in achieving the desired prior austenite grain size and associated torsional fatigue strength in the present invention, but if it exceeds 0.005%, the amount of precipitated AlN increases.
- the Al content is 0.005% or less. Preferably it is 0.003% or less.
- N 0.0050% or less N is an element that combines with Al to form AlN and contributes to the suppression of austenite grain growth in the normalizing step. To suppress this effect, 0.0050% or less It is necessary to. In addition, Preferably it is 0.0035% or less.
- Cr 0.1 to 0.5% Cr is an element that lowers the pearlite transformation temperature, thereby narrowing the lamella spacing of the pearlite and increasing the strength of the pearlite, thereby increasing the torsional fatigue strength. In order to exhibit this effect, the content of 0.1% or more is necessary. On the other hand, if the content exceeds 0.5%, an oxide is formed and remains in the electric seam, so that the electric weldability (ERW) may be deteriorated. Therefore, the Cr content is in the range of 0.1 to 0.5%. In addition, Preferably it is 0.15 to 0.30% of range.
- the above is the basic chemical component of the present invention. Further, for the purpose of improving the strength and fatigue strength, at least one of Ti, B, Mo, W, Nb, V, Ni, Cu, Ca, and REM shown below is used. Can be contained.
- Ti 0.005 to 0.1%
- Ti has an action of fixing N in steel as TiN. However, if it is less than 0.005%, the ability to fix N is not sufficiently exhibited, while if it exceeds 0.1%, the workability and toughness of the steel deteriorate.
- the Ti content is preferably in the range of 0.005 to 0.1%. More preferably, it is in the range of 0.01 to 0.04%.
- B 0.0003 to 0.0050%
- B is an element that improves hardenability. If it is less than 0.0003%, the effect of improving hardenability is not sufficiently exhibited. On the other hand, even if the content exceeds 0.0050%, the effect is saturated, segregates at the grain boundary, promotes intergranular fracture, and deteriorates fatigue resistance.
- the B content is preferably in the range of 0.0003 to 0.0050%. More preferably, it is in the range of 0.0010 to 0.0040%.
- Mo 2% or less Mo is an element that improves hardenability, and is effective in increasing the strength of steel and improving fatigue strength. In order to acquire the effect, containing 0.001% or more is preferable. However, if it exceeds 2%, the workability is remarkably lowered.
- the amount of Mo is preferably 2% or less. More preferably, it is in the range of 0.001 to 0.5%.
- W 2% or less W is effective in improving the strength of steel by forming carbides. In order to acquire the effect, containing 0.001% or more is preferable. However, if it contains more than 2%, unnecessary carbides are precipitated, and the fatigue resistance is lowered and the workability is lowered.
- W content is preferably 2% or less. More preferably, it is in the range of 0.001 to 0.5%.
- Nb 0.1% or less
- Nb is an element that improves hardenability, and also forms carbides and contributes to an increase in strength. In order to acquire the effect, containing 0.001% or more is preferable. However, even if it contains exceeding 0.1%, the effect will be saturated and workability will fall.
- the Nb content is preferably 0.1% or less. More preferably, it is 0.001 to 0.04% of range.
- V 0.1% or less
- V is an element which forms carbides and is effective in increasing the strength of steel and has a temper softening resistance. In order to acquire the effect, containing 0.001% or more is preferable. However, if the content exceeds 0.1%, the effect is saturated and workability is lowered.
- the V content is preferably 0.1% or less. More preferably, it is in the range of 0.001 to 0.5%.
- Ni 2% or less
- Ni is an element that improves hardenability, and is effective in increasing the strength of steel and improving fatigue strength. In order to acquire the effect, containing 0.001% or more is preferable. However, if it exceeds 2%, the workability is remarkably lowered.
- the amount of Ni is preferably 2% or less. More preferably, it is in the range of 0.001 to 0.5%.
- Cu 2% or less Cu is an element that improves hardenability, and is effective for increasing the strength of steel and improving fatigue strength. In order to acquire the effect, containing 0.001% or more is preferable. However, if it exceeds 2%, the workability is remarkably lowered.
- the amount of Cu is preferably 2% or less. More preferably, it is in the range of 0.001 to 0.5%.
- Ca 0.02% or less
- REM 0.02% or less
- Each of Ca and REM has a non-metal inclusion shape in a spherical shape and is subjected to cyclic stress. It is an element effective for reducing the crack starting point at the time of fatigue failure under the use environment, and can be selected and contained as necessary. Such an effect is recognized when both Ca and REM contain 0.0020% or more. On the other hand, if the content exceeds 0.02%, the amount of inclusions is excessively increased, and the cleaning level is reduced. For this reason, when Ca and REM are contained, it is preferable that both Ca and REM be 0.02% or less. When both Ca and REM are used in combination, the total amount is preferably 0.03% or less.
- the balance other than the components described above is Fe and inevitable impurities.
- pearlite has a metal structure of an area fraction of 85% or more, and the total of the ferrite area fraction and the bainite area fraction (including 0) is 15% or less.
- the main structure is pearlite and the area fraction is 85% or more in order to exert the effect of increasing the fatigue crack propagation resistance and improving the fatigue strength by propagating the fatigue crack in zigzag. is there.
- the area fraction of soft ferrite and the area fraction (including 0) of bainite that is hard but does not exhibit the same effect as pearlite exceeds 15%, the effect of improving fatigue strength is To reduce. Therefore, the area fraction of pearlite is 85% or more, and the total of the area fraction of ferrite and the area fraction of bainite (including 0) is 15% or less.
- the prior austenite grain size is 25 ⁇ m or more
- the prior austenite grain size needs to be 25 ⁇ m or more, and if it is less than 25 ⁇ m, the increase in fatigue crack propagation resistance is not sufficient.
- the lamella spacing of pearlite the narrower the pearlite strength is, as is conventionally known.
- the lamella spacing is preferably 170 nm or less. Desirably it is 150 nm or less.
- the steel strip which hot-rolled the steel slab which shows steel composition (mass%) in Table 1 is obtained, this is an ERW steel pipe (outer diameter 89mm, wall thickness 4.7mm) by roll forming (roll forming) and high frequency resistance welding. After that, a steel pipe (outer diameter 45 mm, wall thickness 4.5 mm) reduced in diameter by hot reduction rolling was manufactured. Then, after cold-drawn steel tube (cold draft steel tube) (outer diameter 40 mm, wall thickness 4.0 mm), normalization (920 ° C. ⁇ 10 minutes holding, cooling rate after soaking 0.5) ⁇ 3.0 ° C / s) to obtain a product steel pipe.
- Tensile specimen (JIS No. 12 test piece) was collected in the axial direction from the product steel pipe, and the tensile strength was measured. Moreover, the corrosion which shows an austenite grain boundary was performed about the pipe circumferential direction cross section of the steel pipe, and the austenite particle size was measured. The particle size was measured by taking a picture of 10 fields of view at a magnification of 400 times with an optical microscope (optical microscope), measuring the particle size based on a cutting method (method of section), and taking the average value as a representative value.
- the ERW steel pipes of the examples of the present invention all have small strength variations due to changes in the cooling rate of the normalization, and are excellent in strength stability. Since the austenite grain size is large, the fatigue crack radio resistance is high, and it has a stable high torsional fatigue strength.
- the tensile strength is low in the region where the cooling rate of normalization is slow, and accordingly the torsional fatigue strength is low.
- the tensile strength is smaller than that of the inventive example, but the torsional fatigue strength is lower than that of the inventive example. This is thought to be due to the difference in the prior austenite grain size and the difference in the intensity of pearlite.
- the pipe material of the ERW steel pipe is a hot-rolled steel sheet in the present embodiment
- the present invention is not limited to this, and a cold-rolled steel strip may be used as the pipe material.
- the thing of the form which did not perform hot diameter reduction rolling but used the normal ERW steel pipe as the cold checker tube may be sufficient.
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Abstract
Description
はじめに、本発明の鋼の成分組成を規定した理由を説明する。なお、成分%はすべて質量%を意味する。
Cが0.35%未満では、十分な強度が得られず、要求される耐疲労特性が得られない。一方で、0.55%を超えると、溶接性が悪くなる為、安定した電縫溶接品質が得られない。よって、C量は0.35~0.55%の範囲とする。好ましくは0.40~0.45%の範囲である。
Siは脱酸のために添加する場合もあり、0.01%未満では十分な脱酸効果(deoxidation effect)が得られない。同時に、Siは固溶強化元素(solute strengthening elements)でもあり、その効果を得るためには0.01%以上の含有が必要である。一方で、1.0%を超えると、鋼管の焼入れ性が低下する。Si量は0.01~1.0%の範囲とする。好ましくは0.1~0.4%である。
Mnはパーライト変態を促進させ、また焼入れ性を向上させる元素であり、その効果を得るには1.0%以上の添加が必要である。一方で、3.0%を超えると電縫溶接品質(welding quality of ERW)を低下させ、さらに残留オーステナイト量(amount of residual austenite)が増加し耐疲労特性が低下する。Mn量は1.0~3.0%の範囲とする。好ましくは1.4~2.0%の範囲である。
本発明でPは不可避的不純物であり、その量の上限を0.02%以下とする。Pは連続鋳造時に形成される偏析部(segregation part)に濃化する傾向があり、管素材の熱延鋼板においても残存する。電縫溶接時には鋼帯のエッジ(edges)を突合せアプセット(upset)をかけるため、Pが濃化した偏析部分は管の外表面および内表面に露出する場合があり、この部分に扁平加工(flattening forming)などの二次加工(secondary processing)が付与された場合に割れを生じる危険性がある。したがって、好ましくは0.01%以下である。
本発明でSは不可避的不純物であり、その量の上限を0.01%以下とする。Sはその量が多いと素材の靭性を低下させるほか、鋼中のMnと結合しMnSを形成する。これは熱延工程で長手方向に伸ばされた長い介在物になり、加工性、靭性を低下させることになる。したがって、好ましくは0.005%以下、さらに好ましく0.003%以下である。
Alは本発明において所望の旧オーステナイト粒径とそれにともなうねじり疲労強度を達成する上で重要な元素であるが、0.005%を超えて含有するとAlN析出量が増大し、焼準工程でピン止め効果を発揮するため、オーステナイトの粒成長が抑制され、所望のオーステナイト粒径が得られない。従って、Al量は0.005%以下とする。好ましく0.003%以下である。
Nは、Alと結合しAlNを形成し、焼準工程でのオーステナイトの粒成長の抑制に寄与する元素であり、この効果を抑制するためには0.0050%以下とする必要がある。なお、好ましくは0.0035%以下である。
Crは、パーライト変態温度を低下させる元素であり、これによりパーライトのラメラ間隔が狭小化し、パーライトの強度が上昇するため、ねじり疲労強度が上昇する。この効果を発揮するためには0.1%以上の含有が必要である。一方で、0.5%を超えて含有すると、酸化物を形成しこれが電縫部に残存するため、電縫溶接性(weldability of ERW)が劣化する可能性がある。よって、Cr量は、0.1~0.5%の範囲とする。なお、好ましくは0.15~0.30%の範囲である。
Tiは鋼中のNをTiNとして固定する作用を有する。しかし、0.005%未満ではNを固定する能力が十分に発揮されず、一方で0.1%を超えると鋼の加工性および靭性が低下する。Tiを含有する場合は、Ti量は0.005~0.1%の範囲とすることが好ましい。より好ましくは0.01~0.04%の範囲である。
Bは焼入れ性を向上させる元素である。0.0003%未満では焼入れ性向上効果が十分に発揮されない。一方で、0.0050%を超えて含有しても、その効果は飽和し、粒界に偏析して粒界破壊(intergranular fracture)を促進し耐疲労特性を劣化させる。Bを含有する場合は、B量は0.0003~0.0050%の範囲とすることが好ましい。より好ましくは0.0010~0.0040%の範囲である。
Moは焼入れ性を向上させる元素であり、鋼の強度を高め疲労強度の向上に有効である。その効果を得るためには、0.001%以上の含有が好ましい。しかし、2%を超えて含有すると加工性が著しく低下する。Moを含有する場合は、Mo量は2%以下とすることが好ましい。より好ましくは0.001~0.5%の範囲である。
Wは炭化物を形成することで鋼の強度を向上させるのに有効である。その効果を得るためには、0.001%以上の含有が好ましい。しかし、2%を超えて含有すると不必要な炭化物が析出し、耐疲労特性を低下させ加工性(workability)を低下させることになる。Wを含有する場合は、W量は2%以下とすることが好ましい。より好ましくは0.001~0.5%の範囲である。
Nbは焼入れ性を向上させる元素であるほか、炭化物を形成し強度上昇に寄与する。その効果を得るためには、0.001%以上の含有が好ましい。しかし、0.1%を超えて含有してもその効果は飽和し、加工性が低下する。Nbを含有する場合は、Nb量は0.1%以下とすることが好ましい。より好ましくは0.001~0.04%の範囲である。
Vは炭化物を形成し、鋼の強度を上昇させるのに有効でかつ焼戻し軟化抵抗(temper softening resistance)を有する元素である。その効果を得るためには、0.001%以上の含有が好ましい。しかしながら0.1%を超えて含有するとその効果は飽和し、加工性が低下する。Vを含有する場合は、V量は0.1%以下とすることが好ましい。より好ましくは0.001~0.5%の範囲である。
Niは焼入れ性を向上させる元素であり、鋼の強度を高め疲労強度の向上に有効である。その効果を得るためには、0.001%以上の含有が好ましい。しかし、2%を超えて含有すると加工性が著しく低下する。Niを含有する場合は、Ni量は2%以下とすることが好ましい。より好ましくは0.001~0.5%の範囲である。
Cuは焼入れ性を向上させる元素であり、鋼の強度を高め疲労強度の向上に有効である。その効果を得るためには、0.001%以上の含有が好ましい。しかし、2%を超えて含有すると加工性が著しく低下する。Cuを含有する場合は、Cu量は2%以下とすることが好ましい。より好ましくは0.001~0.5%の範囲である。
Ca、REMは、いずれも非金属介在物(non−metal inclusion)の形態を球状とし、繰り返し応力(cyclic stress)が付与されるような使用環境下での疲労破壊時の割れ起点の低減に有効な元素であり、必要に応じて選択して含有できる。このような効果は、Ca、REMともに0.0020%以上の含有で認められる。一方で、0.02%を超えて含有すると、介在物量が多くなりすぎて清浄度(cleaning level)が低減する。このためCa、REMを含有する場合は、Ca、REMともにそれぞれ0.02%以下とすることが好ましい。Ca、REMの両者を併用する場合には、合計量で0.03%以下とすることが好ましい。
本発明では、パーライトを面積分率(area ratio)で85%以上、フェライトの面積分率およびベイナイトの面積分率(0を含む)の合計を15%以下の金属組織とする。
フェライト層に囲まれた見かけ上のパーライト粒径が大きい方が疲労亀裂の偏向が大きくなり、亀裂伝播抵抗は高くなる。フェライトが旧オーステナイトの粒界に生成するため、旧オーステナイトの粒が大きいほど、みかけ上のパーライト粒径は大きくなる。亀裂伝播抵抗を上昇させるためには旧オーステナイト粒径が25μm以上である必要があり、25μm未満では疲労亀裂伝播抵抗の上昇は十分ではない。
Claims (2)
- 成分組成が、質量%で、C:0.35~0.55%、Si:0.01~1.0%、Mn:1.0~3.0%、P:0.02%以下、S:0.01%以下、Al:0.005%以下、N:0.0050%以下、Cr:0.1~0.5%を含有し、残部Fe及び不可避的不純物からなり、金属組織が、パーライト、フェライトおよびベイナイトからなり、前記パーライトの面積分率を85%以上、前記フェライトの面積分率および前記ベイナイトの面積分率(0を含む)の合計を15%以下とし、旧オーステナイト粒径が25μm以上である電縫鋼管。
- 前記成分組成に加えて、さらに、質量%で、Ti:0.005~0.1%、B:0.0003~0.0050%、Mo:2%以下、W:2%以下、Nb:0.1%以下、V:0.1%以下、Ni:2%以下、Cu:2%以下、Ca:0.02%以下、REM:0.02以下の中から選ばれる1種以上を含有する電縫鋼管。
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JP2014559807A JP5892267B2 (ja) | 2013-01-31 | 2014-01-30 | 電縫鋼管 |
US14/765,206 US20150368768A1 (en) | 2013-01-31 | 2014-01-30 | Electric Resistance Welded Steel Pipe |
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MX2018003854A (es) * | 2015-09-29 | 2018-06-15 | Jfe Steel Corp | Tubo de acero soldado por resistencia electrica para estabilizador hueco de alta resistencia, metodo para la fabricacion de tubo de acero soldado por resistencia electrica para estabilizador hueco de alta resistencia, estabilizador hueco de alta resistencia, y metodo para la fabricacion de estabilizador hueco de alta resistencia. |
KR102010201B1 (ko) * | 2015-12-21 | 2019-08-12 | 닛폰세이테츠 가부시키가이샤 | 아즈롤형 k55 전봉 유정관 및 열연 강판 |
US11168375B2 (en) | 2016-09-21 | 2021-11-09 | Jfe Steel Corporation | Steel pipe or tube for pressure vessels, method of producing steel pipe or tube for pressure vessels, and composite pressure vessel liner |
KR102043524B1 (ko) * | 2017-12-26 | 2019-11-12 | 주식회사 포스코 | 초고강도 열연 강판, 강관, 부재 및 그 제조 방법 |
JP6544497B1 (ja) * | 2018-10-12 | 2019-07-17 | 日本製鉄株式会社 | トーションビーム用電縫鋼管 |
WO2020179737A1 (ja) * | 2019-03-06 | 2020-09-10 | 日本製鉄株式会社 | 熱間圧延鋼板およびその製造方法 |
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