WO2008078917A1 - Aciers haute résistance de classe api-x80 destinés à des tuyaux en spirale présentant moins de modification de la résistance, procédé de production correspondant - Google Patents

Aciers haute résistance de classe api-x80 destinés à des tuyaux en spirale présentant moins de modification de la résistance, procédé de production correspondant Download PDF

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Publication number
WO2008078917A1
WO2008078917A1 PCT/KR2007/006728 KR2007006728W WO2008078917A1 WO 2008078917 A1 WO2008078917 A1 WO 2008078917A1 KR 2007006728 W KR2007006728 W KR 2007006728W WO 2008078917 A1 WO2008078917 A1 WO 2008078917A1
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steel
less
steel plate
strength
rolling
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PCT/KR2007/006728
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English (en)
Inventor
Ki Soo Kim
Jin Ho Bae
Seung Woo Yang
Duk Gyu Kim
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Posco
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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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • 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
    • C22C38/02—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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/02—Hardening by precipitation

Definitions

  • the present invention relates to high strength API-X80 grade hot-rolled thick steels for spiral pipes with less strength changes after pipe forming, and a method for manufacturing the same. More particularly, the present invention relates to steel for linepipes having a strength of API-X80 grade (yield strength of 552 MPa or more), which is formed by suitably controlling contents of niobium (Nb), titanium (Ti), molybdenum (Mo) and nickel (Ni) generally added for low carbon aluminum-killed steel, a ratio between these components, and carbon equivalent, while suitably adjusting heating temperature, reduction ratio, and cooling rate in hot rolling to have fine precipitates in the steel.
  • the present invention also relates to a method for manufacturing the same.
  • the added amount of alloy elements for the API-X80 grade steel is typically 0.47 or more, which generates many defects during steel pipe welding and dis advantageously leads to undesired strength of a pipe through decrease in strength of hot-rolled steel after pipe forming.
  • hot working is performed by low temperature rolling to achieve low temperature toughness, which leads to an increase in rolling load and thus overall load of equipment.
  • hot-rolled steel is formed into a spiral pipe generally by a roll forming process.
  • the steel undergoes deformation, which is accumulated in the steel and inevitably changes mechanical properties of the steel. If the mechanical properties of the steel are deteriorated, the final pipe fails to satisfy strength standard even though the steel satisfies the strength standard prior to pipe forming. Therefore, the steel for pipes must have a low difference in mechanical properties from the pipe, and have optimized microstructure and precipitates that determine the mechanical properties.
  • the aforementioned steel for linepipes is manufactured by steel manufacturing, continuous casting, and hot rolling, in which the hot rolling is generally performed in such a way that a slab prepared by continuous casting and having a thickness of 230 ⁇ 250mm is homogenized at a temperature of 1100- 125O 0 C, hot-rolled in the order of rough rolling and finish rolling to have a desired thickness, and cooled to have desired microstructure.
  • the steel for linepipes can be distinguished from general hot-rolled steel by the rough rolling. Namely, compared to the rough rolling for the general hot-rolled steel performed in consideration of simple thickness reduction, the rough rolling for the linepipe steel should be performed via suitable control of deformation amount, temperature of rough rolling, and cooling conditions, since precipitation of added alloying components begins in this process. In rough rolling of the steel for linepipes, such a control is given for the purpose of finally obtaining fine structure, by which desired low temperature toughness can be ensured.
  • Nb-based precipitates are generally formed in the steel and suppress austenite recrystallization, which occurs in high temperature rough rolling of austenite structure, and which will be defined as non-recrystallization rolling.
  • a total rolling reduction at a precipitation generating temperature or less that is, a non-recrystallization reduction ratio, becomes one of important operation factors. If the Nb-based precipitates are not formed due to non-addition of Nb, strain energy accumulated in the steel by meaningful rolling deformation of the steel can be released by the recrystallization.
  • the austenite structure becomes denser by generation of precipitation during the rough rolling, and thus provides many nucleation sites for ferrite transformation in a cooling process, so that the final steel can have finer microstructure and improved low temperature toughness. Since the microstructure of the linepipe steel manufactured as described above has a great amount of grain boundaries capable of obstructing propagation of cracks, it suppresses propagation of cracks, which can occur during use of the linepipe, improving stability of the linepipe used for carrying gas or petroleum.
  • the present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a method of manufacturing API-X80 grade steel for spiral linepipes, which can sufficiently suppress welding cracks and strength reduction after pipe forming, and which ensures the spiral linepipes have a yield strength of 552 MPa or more.
  • the steel for spiral linepipes according to the present invention is formed by suitably controlling contents of alloying components and ratio therebetween in the steel, thereby lowering frequency of welding defects and strength reduction after pipe forming, while ensuring desired low temperature toughness at a rolling load, which does not burden equipment.
  • the present invention provides a high strength API-X80 grade hot-rolled steel plate, comprising, by weight %,: 0.05-0.075% C; 1.65-1.85% Mn; 0.20-0.35% Si; 0.055-0.085% Nb; 0.02-0.06% Ti; 0.20-0.45% Ni; 0.30% or less Mo; 0.05% or less Al; 0.004-0.008% N; and the balance of Fe and unavoidable impurities, wherein an absolute value of Ti-4C-3.42N-1.5S is controlled in the range of 0.15-0.31 , Ceq represented by the following Equation is 0.45 or less, and microstructure of the steel plate includes 75% or more acicular ferrite and 3% or less pearlite in terms of area fraction.
  • the steel plate further comprises 0.01% or less V.
  • the unavoidable impurities comprise 0.0015 wt% S and 0.01 wt% or less
  • the present invention provides a method of manufacturing a high strength hot-rolled steel plate for linepipes, comprising: reheating a steel slab to l,100 ⁇ l,250°C, the slab comprising, by weight %(wt%), 0.05-0.075% C, 1.65-1.85% Mn, 0.20-0.35% Si, 0.055-0.085% Nb, 0.02-0.06% Ti, 0.20-0.45% Ni, 0.30% or less Mo, 0.05% or less Al, 0.004-0.008% N, and the balance of Fe and unavoidable impurities, wherein an absolute value of Ti- 4C-3.42N-1.5S is in the range of 0.15-0.31, and Ceq represented by the following Equation is 0.45 or less; rough rolling the reheated steel slab at l,100 ⁇ 950°C for re- crystallization reduction and non-recrystallization reduction with a uniform distribution reduction ratio of 14-18%, followed by finish rolling at 760 ⁇ 850°C to prepare
  • the steel slab further comprises 0.01% or less V.
  • the unavoidable impurities comprise 0.0015 wt% S and 0.01 wt% or less
  • Fig. 1 is a graph explaining the Bauschinger effect which is a mechanism of stress- strain characteristics change of material when the material is formed to a pipe by pipe mill;
  • Fig. 2 is a graph of Nb-precipitation behavior related to change in temperature of a steel plate
  • FIG. 3 is microphotographs of microstructure of inventive steel depending on a cooling rate
  • Fig. 4 is a microphotograph of microstructure of Inventive Steel 3;
  • Fig. 5 is a graph depicting a change (drop) in yield strength of steel between before and after pipe forming
  • Fig. 6 is microphotographs showing microstructure of steel depending on a pearlite fraction, and drop of yield strength thereof.
  • Fig. 7 is a graph of percent ductile fracture of inventive steels depending on temperature by drop weight tear test.
  • the present invention relates to a steel plate for spiral steel pipes.
  • Ceq is the indicator of determining the welding properties of a steel plate.
  • An excessive increase in added amount of alloying elements for ensuring the strength of steel plate causes an increase in Ceq, which leads to deterioration of the welding properties.
  • the steel plate is likely to experience undesired defects such as a cold crack. Therefore, it is desirable that the steel plate have Ceq of 0.45 or less.
  • composition of the steel plate is controlled as follows in order to ensure the strength of steel plate and to make it easy to regulate Ceq in the range of the present invention.
  • present invention will be described in detail in view of composition, structure and manufacturing process.
  • Carbon is a fundamental element that is combined with alloying elements to form precipitates in the steel. Such carbon-based precipitation is referred to as “Carbides”, and provides different influence on the strength and toughness of steel depending on size and distribution thereof. Typically, finer precipitations make a higher contribution to improvement in strength of steel and have a lower influence on deterioration of low temperature toughness.
  • the content of carbon less than 0.05% becomes a cause of load in the steel manufacturing process and has little effect in strength improvement.
  • an excessive content of carbon more than 0.75% leads to carbide coarsening and an increase in amount of pearlite, which are ineffective in strength improvement and provide adverse effects to the low temperature toughness.
  • an increase in content of carbon leads to an increase in solid solution temperature of the added alloying elements during reheating, which requires an excessive increase of the reheating temperature.
  • the content of carbon is preferably in the range of 0.05-0.75%.
  • Manganese (Mn) generally combines with sulfur (S) during the steel manufacturing process to prevent red brittleness, and provides a solid solution strengthening effect when added in a predetermined amount or more. Further, Mn serves to retard initiation of transformation into ferrite or pearlite. Therefore, the content of Mn is set in the range of 1.65-1.85% to obtain desired strength of the steel according to the present invention.
  • an excessive added amount of Mn generates large amounts of central segregation in a slab, which leads to deterioration in low temperature toughness and can be advanced to central cracks during pipe working.
  • the upper limit of Mn is set to 1.85%.
  • Silicon (Si) serves to increase the strength of steel by solid solution strengthening.
  • the content of Si is preferably in the range of 0.20-0.35% in consideration of the strength and quality of steel.
  • Niobium (Nb) is an element that provides a more important influence on hot rolling properties of linepipes and mechanical properties of final steel than any of other alloying elements added to the steel. This is attributable to the fact that Nb is dissolved in the steel at a reheating temperature and forms carbonitride precipitates in the region of hot rolling temperature. As can be appreciated from Fig. 2 that shows a representative solid solution curve, the alloying element can be sufficiently utilized when temperature and time are controlled to allow the alloying elements to be sufficiently dissolved during the reheating process, and almost of all Nb exists in a solid solution state in the steel at about 1,200 0 C irrespective of steel kinds.
  • Nb in consideration of metallurgical influence.
  • the content of Nb is preferably in the range of
  • 0.055-0.085% which is a condition capable of providing optimal effects through addition of alloying iron in the compositional range of inventive steel in consideration of the hot rolling process and metallurgical characteristics of inventive steel.
  • the content of Nb is less than 0.055%, the non-recrystallization temperature is lowered.
  • low temperature rolling is required. IHbwever, the low temperature causes a significant increase in deformation resistance of the steel, which increases rolling load above the equipment limit. As a result, the non-recrystallization rolling reduction cannot be sufficiently applied to the steel at the low temperature, making it difficult to ensure the strength and toughness of the steel.
  • the content of Nb exceeding 0.085% can cause brittleness cracks at corners of the steel, making it difficult to process the hot rolled steel into a normal pipe.
  • Titanium (Ti) is a very intensive compound forming element and is precipitated in the sequence of sulfide (TiS), nitride (TiN), and carbide (TiC) with change in temperature of the steel from high temperature to low temperature.
  • Ti is an element that highly influences precipitation hardening of the steel, and is preferably added in the range of 0.02-0.06%.
  • a predetermined amount of Ti combines with manganese sulfide (MnS), which is formed at a very high temperature, to increase the strength of MnS, and a more amount of Ti must combine with sulfides and nitrides precipitated at a high temperature, than what is actually necessary.
  • Ti is preferably added to the steel in the lower limit of 0.02% to obtain the precipitation hardening effect.
  • TiC titanium carbide
  • Ti precipitates have a complex shape like titanium carbonitride (Ti(C 5 N)), and preferably have a fine size in a ferrite phase to ensure an effective increase in strength.
  • Ti also serves to increase the non-recrystallization initiation temperature with an increase in added amount of Ti as in the case of Nb. IHbwever, if the content of Ti exceeds 0.06%, coarsening of TiN occurs to provide adverse effects in view of low temperature toughness.
  • the content of Ti is preferably in the range of 0.02-0.06%.
  • Ni nickel
  • the content of Ni is preferably in the range of 0.20-0.45% to ensure the desired low temperature toughness while preventing an increase in manufacturing costs.
  • Molybdenum (Mo) provides high influence during cooling of the steel, and serves to increase a fraction of acicular ferrite in a polygonal ferrite phase of the steel in the same conditions of hot rolling.
  • the acicular ferrite or the pseudo polygonal ferrite exhibits superior low temperature toughness to the polygonal ferrite, and can be produced via quenching.
  • Mo is added to surmount such insufficient cooling conditions by promoting generation of the acicular ferrite.
  • IHbwever a highly added amount of Mo causes an increase in manufacturing costs and generation of bainite, which leads to deterioration in low temperature toughness.
  • the steel according to the invention preferably comprises 0.30% or less Mo in order to suppress shape defects such as upward or downward bending of the hot rolled steel, which can occur during the hot rolling.
  • Aluminum (Al) is an element used as an oxidizing agent for deoxidation of the steel and is generally added in an Al-killing process. Typically, Al exists in the form of an oxide in the steel and provides a structure refining effect. IHbwever, since a highly added amount of Al degrades cleanliness of the steel, Al is preferably added up to 0.05%.
  • N is an element that suppresses growth of austenite grains and forms TiN precipitates during slab heating, thereby suppressing the austenite grain growth in a welding heat affected zone. Therefore, N is preferably added in an amount of 0.004% or more. IHbwever, since an excessive content of N promotes surface defects on the slab while deteriorating the toughness of the welding heat affected zone, N is preferably added up to 0.008% or less.
  • the steel plate according to the present invention preferably comprises
  • the steel plate preferably further comprises 0.01% or more V.
  • V is an element that provides a similar function to Nb and effectively increases the strength of steel through formation of precipitates at low temperature.
  • Nb is preferentially added to the steel.
  • Fbwever since the effect as described above can be further improved by addition of Nb along with V, V can be further added to the steel.
  • Nb and V are added for the purpose of increasing the strength of steel through formation of precipitates in the form of carbides.
  • the content of V preferably has an upper limit of 0.1%.
  • the steel plate may comprise unavoidable impurities, of which kinds are too numerous to be individually referred to.
  • sulfur (S) and phosphorus (P) be restricted as much as possible. Therefore, it is more effective in obtaining the properties of steel according to the invention to set preferable added amounts of S and P.
  • the preferable added amount of each component and the reason thereof will be described.
  • S Sulfur
  • S is a detrimental element in view of mechanical properties and is required to be suppressed in content as much as possible in the steel.
  • S can be allowed up to 0.0015%, which is considered not problematic in mass production of the steel.
  • Phosphorus (P) is an element that causes central or grain boundary segregation in the steel.
  • P facilitates brittleness fracture at the center of the steel, it is also necessary to suppress the content of P as much as possible in the steel.
  • load of the steel manufacturing process also increases as in the case of S to manage the content of P to an excessively low content, P is allowed up to 0.01% in the present invention.
  • an absolute value of Ti-4C-3.42N-l.5S is in the range of 0.15-0.31.
  • Ti, C, N, and S means wt% of respective elements.
  • the relation of Ti- 4C-3.42N-1.5S can be used as an index indicating that except for the amount of Ti contained in TiS and TiN precipitates at high temperature, Ti dissolved in the steel during the hot rolling is formed to TiC precipitated at a relatively low temperature. Since the Ti-based precipitates formed in the steel at the low temperature are fine precipitates, they are very effective in improving the strength of steel without reducing the low temperature toughness.
  • An index of Ti amount to remain in the steel for formation of the fine precipitates is advantageously controlled to 0.15 or more, Fbwever, since an excessively high index of Ti amount is likely to cause coarsening of TiN and TiS, which are stable at high temperature, and deteriorates the low temperature toughness, the index of Ti amount is preferably 0.31 or less.
  • the steel plate for linepipes according to the invention has suitable conditions for realizing high strength and good welding properties. Additionally, to ensure high strength of API-X80 grade or more, it is desirable that the steel plate have microstructure as follows.
  • the steel plate when changing rolling and cooling conditions of the steel plate, the steel plate can be formed with various microstructure phases.
  • inventors of the present invention found that, when the steel plate comprises acicular ferrite as a main microstructure phase, it is preferable to ensure the strength and toughness of the steel and a pipe after pipe forming.
  • the microstructure of the steel plate preferably comprises acicular ferrite in an area fraction of 75% or more. Since hard microstructure such as bainite or martensite significantly reduces the toughness despite an increase in strength of the steel plate, they are unfavorable for the steel plate of the present invention.
  • the acicular ferrite is formed as the main phase of the microstructure in the steel plate and pearlite as a secondary phase is preferably suppressed to minimize decrease in strength after the pipe forming. At this time, the fraction of pearlite is preferably 3% or less.
  • Fig. 1 shows stress-strain curves of a UOE steel pipe and a spiral steel pipe when performing the pipe forming.
  • the UOE steel pipe is subjected to a series of processes such as U-shape working, pipe forming, pipe expansion, etc., in which the UOE steel pipe undergoes a temporary reduction in yield strength during the pipe forming and an increase in yield strength during the pipe expansion to a desired diameter.
  • the reduction in yield strength during the pipe forming is based on the so-called Bauschinger effect, by which although dislocations are piled up to act as obstacles against deformation in the steel by application of stress during the pipe forming, they are converted into a cause of encouraging the deformation when stress is applied to the steel opposite the previous stress. Therefore, the yield strength of a steel pipe is generally lowered after the pipe forming. However, in manufacture of the UOE steel pipe, the pipe expansion is performed after the pipe forming to have a desired diameter, and increases the strength through work hardening caused thereby.
  • the inventors of the present invention found that pearlite structure in a steel plate is a main cause of lowering the yield strength after the pipe forming.
  • a strength reduction degree related to the Bauschinger effect increases substantially in proportion to an area fraction of pearlite in the steel.
  • the area fraction of pearlite structure in the steel must be restricted to 3% or less to suppress the strength reduction degree so as not to cause a severe problem.
  • the method of manufacturing the steel plate according to the present invention comprises reheating a steel slab, hot rolling the reheated steel slab to provide a hot- rolled steel plate, and cooling the steel plate to a coiling temperature. Suitable conditions for the respective processes will be described as follows. [98]
  • a steel slab having the composition as described above is reheated to change columnar structure, formed during continuous casting, into structure suitable for hot rolling while dissolving precipitates again in the steel slab.
  • the reheating temperature is preferably in the range of 1,100-1,250 0 C. If the reheating temperature is less than 1,100 0 C, the alloying elements precipitated during the continuous casting cannot be sufficiently dissolved, thereby making it difficult to obtain desired metallurgical effects such as strength increase and the like. If the reheating temperature is above 1,200 0 C, the slab is likely to have coarsened austenite grains, some of which remain in a final steel plate and deteriorate the properties of steel plate. Therefore, according to the invention, the steel slab is heated for 180 minutes or more in the temperature range as suggested above so as to prevent deterioration of hot rolling properties due to insufficient heating in the interior of the slab.
  • the steel slab is subjected to rough rolling at 1,100-950 0 C. If the rough rolling is performed at too high temperatures, compounds of elements such as Nb are not sufficiently precipitated, thereby making it difficult to achieve sufficient suppression of austenite grain growth. Conversely, if the rough rolling is performed at too low temperatures, a rolling load is increased above design load for equipment. Further, since austenite recrystallization and non- recrystallization are sequentially occurred in accordance with temperature decrease in this process, the reduction ratio per pass is set in the range of 14-18%. If the reduction ratio is 14% or less, deformation does not sufficiently propagate into the center of the steel plate, causing non-uniform properties in the thickness direction.
  • the rolling load becomes excessively increased to provide a problem in view of equipment load or to provide an unfavorable shape to the steel plate, which makes it difficult to apply continuous rolling deformation to the steel plate.
  • Finish rolling temperature 760-850 0 C
  • Finish rolling is preferably performed at 760-850 0 C. As in the rough rolling, if the finish rolling is performed at too low temperatures, finish rolling mills are likely to experience an increased load. Conversely, if the finish rolling is performed above 850 0 C, it is difficult to obtain desired toughness and strength.
  • Cooling rate 10-20 °C/sec
  • the hot-rolled steel plate is cooled to 600 0 C or less at a cooling rate of 10-20 °C/sec.
  • a suitable cooling rate is one of important factors that determine final microstructure of steel. As can be appreciated from Fig. 3 showing a relationship between cooling rate and microstructure, an excessively low cooling rate leads to a soft phase, whereas an excessively high cooling rates leads to a decrease in ductility. Therefore, according to the invention, the cooling rate is set in the range as described above. During such a cooling process, if a coiling start temperature is maintained at 600 0 C or more, the steel plate is in the pearlite transformation nose position, and formed with local pearlite. Thus, it is necessary to maintain the coiling temperature below this temperature in order to obtain a desired phase.
  • the steel plate After being cooled as above, the steel plate is coiled.
  • the cooling rate and the coiling temperature are set based on the content of Mo set to 0.30%, and if the content of Mo exceeds 0.30%, acicular ferrite can be easily formed at a relatively slow cooling rate.
  • the steel plate according to the invention When manufactured in the conditions as described above, the steel plate according to the invention satisfies desired properties of final hot-roll thick steel products and is characterized in that it has pearlite structure in an area fraction of 3% or less. Therefore, the present invention can suppress a strength reduction degree after the pipe forming to be 40 MPa or less, and thus ensures API-X80 grade strength of pipes.
  • Table 1 shows compositions of various steel samples, (unit : wt%)
  • the steel slab was reheated at 1,180 0 C for 180 minutes in nitrogen atmosphere, followed by hot-rolling in conditions as shown in the following Table 2
  • the hot-rolling conditions shown in Table 2 were conditions that can be applied to conventional linepipe steel for low temperature toughness, in which all of the steel samples were rolled to the same thickness of 16.9 mm and cooling rates were obtained by dividing a difference between rolling finish temperature and coiling temperature by a cooling period.
  • Table 3 shows an average of mechanical properties measured twice in a direction of 30 degrees from a rolling direction of the respective steel samples that had the compositions in Table 1 and were manufactured in the conditions for hot rolling in Table 2 Since a hot-rolled steel plate is generally used to provide a spiral steel pipe, the mechanical properties of the pipe in a circumferential direction are important for such a spiral steel pipe and correspond to values in the direction of 30 degrees from a rolling direction of a steel plate prior to pipe processing. Since the hot-rolled steel for linepipes is hot-rolled at a lower temperature than that of other steel, the steel for linepipe is likely to be bent upward, that is, upward bending, which makes it difficult to perform the hot-rolling and deteriorates workability. Other steel samples except for conventional steel (Steel No. 4) did not exhibit upward bending during the hot-rolling.
  • the suitable mechanical properties can be obtained by providing a high area fraction of acicular ferrite while suppressing a fraction of secondary phase such as pearlite as much as possible.
  • the strength reduction after pipe forming is determined by the fraction of secondary phase, and changes in mechanical properties for Inventive Steel 3 and Comparative Steel 8 after the pipe forming are shown in Fig. 5.
  • results of Fig. 5 are closely related to generation of the secondary phase, that is, pearlite, and show that, when steel has 75% or more acicular ferrite and 3% or less pearlite, it is possible to prevent a significant reduction in strength of the steel.
  • Fig. 6 shows a fraction of pearlite structure determining strength reduction after pipe forming, and a strength reduction degree.
  • the fraction of secondary phase was measured using an image analysis apparatus after picral etching for observing only the secondary phase, that is, pearlite.
  • the conventional steel had a high Ceq and a low fraction of secondary phase leading to a lower strength reduction degree than any other steels after the pipe forming, it was inferior to the inventive steels in view of op- erability and easiness in manufacture.
  • inventive steels with the strength reduction degree of 40 MPa or less after the pipe forming can be more advantageously used to manufacture the linepipes in view of op- erability and easiness in manufacture than any other steels.
  • Fig. 7 is a graph depicting low temperature of inventive steels by drop weight tear test. As can be seen from the graph, the inventive steels satisfy a condition for percent ductile fracture of 85% or more to -20 0 C, as is required for a pipe.
  • the present invention can provide API-X80 grade steel for linepipes, of which strength reduction degree after pipe forming is 40 MPa or less, and which ensures high strength and low temperature toughness at the same time.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

La présente invention concerne des aciers haute résistance épais laminés à chaud de classe AIP-X80 destinés à des tuyaux en spirale ayant une moindre variation de la résistance après la formation des tuyaux et un procédé de production correspondant. Pour former cet acier, on sélectionne de manière appropriée, les teneurs en Nb, Ti, Mo et Ni, un rapport entre ces constituants, et un équivalent carbone tout en sélectionnant de manière appropriée la température de chauffage, le taux de réduction et la vitesse de refroidissement dans le laminage à chaud afin d'obtenir des précipités fins. L'acier comprend de 0,05 à 0,075% de C, de 1,65 à 1,85% de Mn, de 0,20 à 0,35% de Si, de 0,055 à 0,085% de Nb, de 0,02 à 0,06% de Ti, de 0,20 à 0,45% de Ni, 0,30% ou moins de Mo, 0,05% ou moins de Al, de 0,004 à 0,008 de N, le reste étant du fer et les impuretés inévitables. Une valeur absolue de Ti-4C-3.42N-1.5S est comprise entre 0,15 et 0,31, Ceq est représenté par l'équation suivante ayant pour valeur maximale 0,45 et la microstructure de l'acier contient au moins 75% de ferrite aciculaire et au maximum 3% de perlite en termes de fraction de surface. Ceq = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
PCT/KR2007/006728 2006-12-26 2007-12-21 Aciers haute résistance de classe api-x80 destinés à des tuyaux en spirale présentant moins de modification de la résistance, procédé de production correspondant WO2008078917A1 (fr)

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EP2116624A1 (fr) * 2007-03-01 2009-11-11 Nippon Steel Corporation Tôle d'acier laminée à chaud hautement résistante pour tubes de canalisation, qui présente une excellente ténacité à basse température, et son procédé de production
CN102851587A (zh) * 2012-09-06 2013-01-02 江苏沙钢集团有限公司 抗变形x80-x100管线钢板及其制造方法
JP2013136827A (ja) * 2011-11-28 2013-07-11 Jfe Steel Corp 脆性き裂伝播停止特性に優れた構造用高強度厚鋼板およびその製造方法
CN103397260A (zh) * 2013-07-30 2013-11-20 首钢总公司 一种薄规格高韧性x80热轧钢板及其生产方法
CN103981460A (zh) * 2014-05-30 2014-08-13 秦皇岛首秦金属材料有限公司 高韧性x80弯管用热轧平板钢及其生产方法
EP2762581A4 (fr) * 2011-09-29 2015-05-20 Jfe Steel Corp Tôle en acier laminée à chaud, et procédé de fabrication de celle-ci
EP2762584A4 (fr) * 2011-09-29 2015-05-27 Jfe Steel Corp Tôle en acier laminée à chaud, et procédé de fabrication de celle-ci
WO2015088040A1 (fr) * 2013-12-12 2015-06-18 Ｊｆｅスチール株式会社 Tôle d'acier et procédé permettant de fabriquer cette dernière
CN108998742A (zh) * 2018-08-08 2018-12-14 南京钢铁股份有限公司 一种极低屈服强度钢板及其制造方法
US10301698B2 (en) 2012-01-31 2019-05-28 Jfe Steel Corporation Hot-rolled steel sheet for generator rim and method for manufacturing the same
US10316385B2 (en) 2014-03-31 2019-06-11 Jfe Steel Corporation High-tensile-strength steel plate and process for producing same
WO2020237976A1 (fr) * 2019-05-28 2020-12-03 南京钢铁股份有限公司 Acier structural à structure aciculaire superfine et son procédé de production
EP3730658A4 (fr) * 2017-12-24 2021-03-03 Posco Matériau en acier pour tuyau en acier à haute résistance et faible limite apparente d'élasticité ayant une excellente ténacité à basse température et procédé de fabrication s'y rapportant
CN115109992A (zh) * 2021-03-22 2022-09-27 宝山钢铁股份有限公司 一种热成型性良好的压力容器用钢板及其制造方法
CN115233099A (zh) * 2022-07-13 2022-10-25 首钢集团有限公司 一种x52级别管线用钢板及其制备方法

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CN107099745B (zh) * 2017-04-01 2019-12-27 江阴兴澄特种钢铁有限公司 高碳当量低温高韧性的x80弯管用管线钢板及其制造方法
CN111716077A (zh) * 2019-03-20 2020-09-29 天津市同鑫泰钢管制造有限公司 一种长距离油气输送用防裂螺旋钢管及其制造工艺

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JPH08209287A (ja) * 1995-02-03 1996-08-13 Nippon Steel Corp 低降伏比を有する低温靱性に優れた高強度ラインパイプ用鋼
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US8562762B2 (en) 2007-03-01 2013-10-22 Nippon Steel & Sumitomo Metal Corporation High strength hot rolled steel products for line-pipes excellent in low temperature toughness and production method of the same
EP2116624A1 (fr) * 2007-03-01 2009-11-11 Nippon Steel Corporation Tôle d'acier laminée à chaud hautement résistante pour tubes de canalisation, qui présente une excellente ténacité à basse température, et son procédé de production
EP2116624B1 (fr) * 2007-03-01 2017-02-22 Nippon Steel & Sumitomo Metal Corporation Tôle d'acier laminée à chaud hautement résistante pour tubes de canalisation, qui présente une excellente ténacité à basse température, et son procédé de production
US9574254B2 (en) 2011-09-29 2017-02-21 Jfe Steel Corporation Hot-rolled steel sheet and method for producing same
EP2762581A4 (fr) * 2011-09-29 2015-05-20 Jfe Steel Corp Tôle en acier laminée à chaud, et procédé de fabrication de celle-ci
EP2762584A4 (fr) * 2011-09-29 2015-05-27 Jfe Steel Corp Tôle en acier laminée à chaud, et procédé de fabrication de celle-ci
US9057123B2 (en) 2011-09-29 2015-06-16 Jfe Steel Corporation Hot-rolled steel sheet and method for producing same
JP2013136827A (ja) * 2011-11-28 2013-07-11 Jfe Steel Corp 脆性き裂伝播停止特性に優れた構造用高強度厚鋼板およびその製造方法
US10301698B2 (en) 2012-01-31 2019-05-28 Jfe Steel Corporation Hot-rolled steel sheet for generator rim and method for manufacturing the same
CN102851587A (zh) * 2012-09-06 2013-01-02 江苏沙钢集团有限公司 抗变形x80-x100管线钢板及其制造方法
CN102851587B (zh) * 2012-09-06 2014-02-12 江苏沙钢集团有限公司 抗变形x80-x100管线钢板
CN103397260A (zh) * 2013-07-30 2013-11-20 首钢总公司 一种薄规格高韧性x80热轧钢板及其生产方法
CN103397260B (zh) * 2013-07-30 2015-08-26 首钢总公司 一种薄规格高韧性x80热轧钢板及其生产方法
JP5950045B2 (ja) * 2013-12-12 2016-07-13 Ｊｆｅスチール株式会社 鋼板およびその製造方法
WO2015088040A1 (fr) * 2013-12-12 2015-06-18 Ｊｆｅスチール株式会社 Tôle d'acier et procédé permettant de fabriquer cette dernière
US10316385B2 (en) 2014-03-31 2019-06-11 Jfe Steel Corporation High-tensile-strength steel plate and process for producing same
CN103981460A (zh) * 2014-05-30 2014-08-13 秦皇岛首秦金属材料有限公司 高韧性x80弯管用热轧平板钢及其生产方法
EP3730658A4 (fr) * 2017-12-24 2021-03-03 Posco Matériau en acier pour tuyau en acier à haute résistance et faible limite apparente d'élasticité ayant une excellente ténacité à basse température et procédé de fabrication s'y rapportant
US11396689B2 (en) 2017-12-24 2022-07-26 Posco Steel material for low yield ratio, high-strength steel pipe having excellent low-temperature toughness, and manufacturing method therefor
CN108998742A (zh) * 2018-08-08 2018-12-14 南京钢铁股份有限公司 一种极低屈服强度钢板及其制造方法
WO2020237976A1 (fr) * 2019-05-28 2020-12-03 南京钢铁股份有限公司 Acier structural à structure aciculaire superfine et son procédé de production
CN115109992A (zh) * 2021-03-22 2022-09-27 宝山钢铁股份有限公司 一种热成型性良好的压力容器用钢板及其制造方法
CN115109992B (zh) * 2021-03-22 2023-09-12 宝山钢铁股份有限公司 一种热成型性良好的压力容器用钢板及其制造方法
CN115233099A (zh) * 2022-07-13 2022-10-25 首钢集团有限公司 一种x52级别管线用钢板及其制备方法

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