Classifications

• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C23/00—Extruding metal; Impact extrusion
  • B21C23/002—Extruding materials of special alloys so far as the composition of the alloy requires or permits special extruding methods of sequences
• • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
  • C21D9/085—Cooling or quenching
• • 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
  • C22C38/04—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
  • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a seamless steel pipe suitable for structural members such as cylinders, bushes, booms, and mechanical members such as shafts, and a method for manufacturing the same.
• Patent Document 1 discloses that a micro-structure of fine and uniform ferrite and cementite having a ferrite grain size of 2 ⁇ m or less is obtained by subjecting a raw tube having a specific composition to a combination of drawing and tilt rolling in a specific temperature range.
• a technique for producing a steel pipe having a structure and high strength and excellent ductility and toughness is disclosed.
• Patent Document 2 discloses an optimum structure capable of achieving both high strength and high toughness across the entire plate thickness direction even in an environment where the cooling rate of the outer surface and the inner surface is different due to accelerated cooling only from the outer surface. Techniques for generating are disclosed.
• Patent Document 3 discloses a steel pipe having a fine metal structure that can achieve both high strength and high toughness by using intragranular transformation by optimizing the addition amount of Al and Ti, and further producing by accelerated cooling after seamless rolling. Is disclosed. However, in this technique, it is necessary to reduce the amount of Al in order to utilize intragranular transformation, and the cost of deoxidation increases.
• Patent Document 4 discloses a steel pipe mainly made of Cr-added steel and having a metal structure of a self-tempered martensite single structure or a mixed structure with lower bainite for the purpose of inexpensively manufacturing a steel pipe for machine structural members. Is disclosed.
• Self-tempered martensite is a structure in which austenite phase has undergone martensitic transformation during accelerated cooling, and fine cementite is precipitated in the lath by cooling after stopping the accelerated cooling.
• the present invention has been made in view of the above situation, and is particularly suitable for structural members such as cylinders, bushes, booms, and mechanical members such as shafts, and has high strength, high toughness and excellent weldability. It is another object of the present invention to provide a seamless steel pipe for machine structure that can suppress the occurrence of surface flaws, and to provide a method for inexpensively manufacturing the seamless steel pipe for machine structure by appropriate heat treatment.
• the present inventors have studied cost reduction by omitting the heat treatment step of the steel pipe for the steel pipe having a component composition to which Cr is not added.
• we focused on and examined steel pipes with accelerated cooling (steel pipes manufactured without heat treatment after accelerated cooling).
• the grain size of the prior austenite in the structure of the steel pipe with accelerated cooling is about 100 ⁇ m, and it is about 20 to 30 ⁇ m in a steel pipe (hereinafter referred to as “QT steel pipe”) subjected to quenching and tempering (hereinafter referred to as “QT steel pipe”).
• the grain size of the structure of the steel pipe with accelerated cooling is coarse compared to the grain size of the structure of the QT steel pipe, and it has been considered that strength and toughness equal to or higher than that of the QT steel pipe cannot be secured.
• the seamless steel pipe of the present invention controls ⁇ , which is an index of hardenability, in an appropriate range, and ensures weldability. Furthermore, Pcm, which is an index of weldability, is controlled within an appropriate range.
• % means “% by mass”.
• C is an element that is extremely effective for improving the strength. In order to obtain the target strength, it is necessary to add 0.03% or more of C.
• the C content is limited to 0.03 to 0.20%.
• the C content is preferably 0.07% or more.
• the C content is preferably 0.15% or less.
• Si is a deoxidizing element and is an element that contributes to improvement in strength. In order to obtain the effect of addition, it is necessary to add 0.01% or more of Si. In order to improve the strength, the amount of Si is preferably 0.10% or more.
• Mn is an element that promotes the formation of a low-temperature transformation structure, and is effective for improving the balance between strength and low-temperature toughness. In order to obtain this effect, it is necessary to add 0.80% or more of Mn. However, if the amount of Mn is more than 3.00%, the low temperature toughness may be impaired, so 3.00% is made the upper limit. In order to improve the balance between strength and low temperature toughness, the preferable range of the amount of Mn is 1.50 to 2.40%.
• P and S are impurities, and if contained excessively, toughness is lowered and weldability is lowered. Therefore, the upper limits of the P and S contents are 0.020% and 0.0080%, respectively.
• the content of P and S is preferably less, and more preferably 0.015% or less and 0.0050% or less, respectively. Since it is preferable not to contain P and S, a lower limit is not prescribed
• Al is a strong deoxidizing element, and more than 0.010% is added from the viewpoint of deoxidation cost.
• the upper limit is limited to 0.050%.
• the upper limit of the Al content is more preferably 0.035%.
• N is an impurity, and if it exceeds 0.0080%, coarse TiN is generated and the toughness is lowered, so the upper limit is limited to 0.0080%.
• the N content is preferably less than 0.0060%, more preferably 0.0050% or less. Since it is preferable not to contain N, the lower limit is not particularly defined. However, if the N content is less than 0.0010%, the production cost increases, so 0.0010% is preferably set as the lower limit.
• O is contained in an amount exceeding 0.0050%, a coarse oxide is generated and the low temperature toughness is impaired, so the upper limit is made 0.0050%. Since it is preferable not to contain O, a minimum in particular is not prescribed
• B may be further added to the steel of the present invention. B is an element that enhances hardenability and contributes to toughening of steel. In order to obtain the effect, 0.0001% or more of B is preferably added. On the other hand, when the addition amount of B is more than 0.0030%, precipitates such as BN are generated, and the hardenability may be lowered.
• Cu and Mo are each preferably added in an amount of 0.05% or more.
• Cu and Mo may impair weldability when the addition amount exceeds 1.00% and 1.50%, respectively.
• Cu may be generated when Cu is added alone, Cu is preferably added simultaneously with Ni.
• Ti, Nb, and V may be added. These are elements that improve the strength of steel by precipitation strengthening.
• Ti is preferably contained in an amount of 0.005% or more in order to precipitate and strengthen steel.
• N is an impurity and improve toughness, it is preferable to add 0.010% or more of Ti.
• the toughness may be reduced by the precipitation of coarse Ti oxides, so the upper limit is preferably made 0.050%.
• the upper limit of Ti amount 0.035% or less.
• Nb is an element that produces precipitates such as carbides and nitrides, suppresses recrystallization of austenite during rolling and refines the structure, and also increases hardenability and is effective in strengthening steel. . If the amount of Nb exceeds 0.050%, coarse Nb precipitates are produced and the toughness may be deteriorated, so the upper limit is made 0.050%. In order to obtain the effect of Nb addition, 0.005% or more is preferably added.
• the contents of Ca, Mg, and REM exceed 0.0040%, 0.0010%, and 0.005%, respectively, inclusions increase excessively and ductility deteriorates. .0040%, 0.0010%, and 0.005%.
• the lower limits of the contents of Ca, Mg, and REM are 0.0005%, 0.0005%, and 0.0001%, respectively.
• the balance of the above elements is Fe and inevitable impurities. Inevitable impurities include Sn, Bi and the like mixed from scrap.
• you may contain Zr, Ta, etc. which are added as needed at the time of deoxidation in the range which does not impair the characteristic of this invention.
• the seamless steel pipe of the present invention is characterized by suppressing the formation of upper bainite and improving toughness.
• ⁇ is an index of the hardenability of steel, and the element symbol of the formula (1) represents the content (% by mass) of each element.
• ⁇ 2.7C + 0.4Si + Mn + 0.45Ni + Mo (1)
• Pcm obtained by the following formula (2) is limited to 0.15 to 0.30.
• the element symbol of Formula (2) represents content (mass%) of each element.
• Pcm C + Si / 30 + (Mn + Cu) / 20 + Ni / 60 + Mo / 15 + V / 10 (2) If Pcm is smaller than 0.15, the required strength cannot be obtained, and the weldability deteriorates. When Pcm exceeds 0.30, the low temperature toughness deteriorates, and further, the weldability deteriorates. A more preferable range of Pcm is more than 0.20 to 0.30. When the selective element included in the formulas (1) and (2) is not added, the content of the element is calculated as 0.
• the difference between Equation (1) and Equation (3) is the coefficient of Mo.
• the metal structure of the seamless steel pipe of the present invention consists of fresh martensite.
• Fresh martensite is a lath-like structure and is different from tempered martensite and bainite in that cementite is not observed by observation with an optical microscope. Since the seamless steel pipe of the present invention is manufactured while being cooled, tempered martensite is not included. Moreover, the seamless steel pipe of this invention suppresses the production
• Fresh martensite is a structure in which austenite is transformed by cooling, and the toughness decreases as the particle size of prior austenite increases.
• the heating temperature of the steel slab is high, and the accumulated strain amount introduced by drilling and rolling cannot be secured, so it is difficult to make the prior austenite fine.
• the particle size of the prior austenite is 50 ⁇ m or more. This is because in order to reduce the grain size of the prior austenite to less than 50 ⁇ m without performing QT treatment, it is necessary to perform piercing and rolling at a low temperature, which increases the manufacturing cost. On the other hand, if the particle size of the prior austenite is coarse, the toughness is lowered.
• the temperature of the steel pipe at the start of accelerated cooling is too high, the austenite grains become coarse and the toughness may be lowered. Therefore, the temperature is 950 ° C. or less, and preferably 900 ° C. or less. Moreover, in order to suppress the ferrite transformation from a crystal grain boundary, the temperature of the steel pipe at the time of starting accelerated cooling shall be 750 degreeC or more. If the cooling rate of accelerated cooling is too slow, upper bainite is generated.
• the upper bainite is a bainite that is generated at a relatively high temperature and contains a large amount of island martensite, which is a local embrittlement phase, resulting in a decrease in toughness.
• the accelerated cooling rate is 10 to 50 ° C./second.
• the accelerated cooling rate means an average cooling rate from the start of accelerated cooling to the stop of cooling. Even if the cooling stop temperature is too high, upper bainite may be generated, and it is preferable to stop the accelerated cooling at 400 ° C. or lower, more preferably 250 ° C. or lower.
• the cooling method can be arbitrarily selected from a method in which water is directly applied to the outer surface of the steel pipe, a method in which water is applied in a tangential direction of the outer periphery of the steel pipe, and mist cooling.
• the steel pipe having the component composition of the present invention By accelerating and cooling the steel pipe having the component composition of the present invention at an appropriate cooling rate, a metal structure composed of fresh martensite in which the formation of upper bainite is suppressed is obtained.
• the steel pipe having the component composition of the present invention has a prior austenite grain size of 50 to 200 ⁇ m, it is not necessary to lower the rolling temperature or utilize intragranular transformation. Therefore, according to the present invention, it is possible to manufacture a steel pipe excellent in strength and toughness without increasing the cost.
• the metal structure is classified into fresh martensite, upper bainite, lower bainite, pearlite, and ferrite. did.
• the particle size of the prior austenite was measured based on JIS G 0551.
• the grain size of prior austenite refers to the grain size of the structure (transformation at high temperature) before transformation to martensite.
• the particle size of the prior austenite does not change even after transformation to martensite, so it can be measured even after transformation.
• the tensile test was performed using an arc-shaped JIS No. 12 tensile test piece, and the yield strength and tensile strength were measured. Evaluation of toughness was based on JIS Z 2242, and a Charpy test was performed at ⁇ 20 ° C.
• the metal structure after accelerated cooling is an upper bainite and a lower bainite.
• the strength is weak and the toughness is inferior.
• No. No. 16 is inferior in hardenability, with B exceeding the upper limit specified in the present invention and ⁇ being 2.11.
• the metal structure after accelerated cooling becomes upper bainite and pearlite.
• the strength is weak and the toughness is inferior.
• No. 17 since C exceeds the upper limit defined in the present invention, the metal structure after accelerated cooling is fresh martensite, but the strength is too high and the toughness is lowered.
• No. 19 is a steel pipe manufactured using steel G whose component composition, ⁇ , and Pcm are within the scope of the present invention, but because the cooling start temperature is high, the metal structure becomes upper bainite and ferrite, and the strength is weak. Toughness is also poor.
• No. 20 is a steel pipe manufactured using steel G, since the cooling start temperature is high, the average grain size of prior austenite was increased, and as a result, toughness was reduced. No.
• 21 is a steel pipe manufactured using steel G, but because the cooling rate is slow, the metal structure becomes pearlite and ferrite, and the average grain size of the prior austenite becomes small, resulting in low strength and high toughness. Somewhat inferior.
• steel that satisfies the component composition, ⁇ , and Pcm defined in the present invention is manufactured by the manufacturing method of the present invention, so that it has the same toughness as a QT steel pipe and is not cooled by QT treatment. It becomes possible to manufacture a steel pipe.
• a seamless steel pipe having a toughness equivalent to or equal to or better than a QT steel pipe which is particularly suitable for structural members such as cylinders, bushes, and booms and mechanical members such as shafts, can be manufactured at low cost. Because it becomes possible, the contribution to the automobile industry, the machine industry, etc. is great.

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• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
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• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
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• Heat Treatment Of Steel (AREA)

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• 2011
  • 2011-03-03 KR KR1020127022016A patent/KR101471730B1/ko active IP Right Grant
  • 2011-03-03 CN CN2011800123186A patent/CN102782173A/zh active Pending
  • 2011-03-03 CN CN201410152807.7A patent/CN103924155B/zh not_active Expired - Fee Related
  • 2011-03-03 WO PCT/JP2011/055562 patent/WO2011108764A1/ja active Application Filing
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