Classifications

• • 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
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the present invention relates to a high-strength hot-rolled steel sheet for spiral pipes using a hot coil having excellent low-temperature toughness and a method for producing the same.
• steel pipes for line pipes can be categorized as seamless steel pipes, UOE steel pipes, ERW steel pipes and spiral steel pipes according to their manufacturing process, and they can be selected according to their use, size, etc.
• Each has the feature that it is commercialized as a steel pipe by forming a plate-shaped steel plate / steel strip into a tubular shape and then seaming it by welding.
• these welded steel pipes can be classified according to whether they use hot coils or plates.
• the former are ERW and spiral steel pipes, and the latter are UF steel pipes.
• the latter UOE steel pipe is generally used for high-strength, large-diameter, and thick-walled applications.
• the cost of power and the delivery of ERW and spiral steel pipes made of the former hot coil in terms of delivery time The demand for high strength, large diameter, and thick wall is increasing.
• a water-cooled stop-type direct quenching method is a characteristic of the plate manufacturing process.
• IDQ Interrupted D irect Quench
• hot coil which is the material of ERW steel pipe and spiral steel pipe, has a winding process as a feature of the process, and it is difficult to wind up thick materials at low temperature due to the restriction of the installation capacity of the coiler. Therefore, it is impossible to stop the low-temperature cooling necessary for strengthening quenching. Therefore, grilled It is difficult to secure strength by strengthening insertion.
• the inclusions are spheroidized by adding C a—S i during milling, and N b, T i, Mo,
• V which has the effect of crystal grain refinement, is added, and in order to maintain strength by using the microstructure as a basic ferrite or a fuzzy ferrite, low-temperature rolling and low-temperature milling are performed.
• the tensile strength in the relevant direction of the steel sheet is 585 MPa or more and — If the ductile fracture surface ratio in the direction corresponding to the circumferential direction of the pipe in the DWT T test at 20 is 90% or more of that in the hot coil width direction— DW at 0
• the ductile fracture surface ratio in the pipe circumferential direction in the TT test is 70% or more, and the strength-toughness balance satisfies the required characteristics as a spiral steel pipe for line pipe applications.
• the present invention can not only withstand the use even in regions where severe low temperature toughness is required, but also for thick, for example, high-strength spiral pipes with a thickness of 14 mm or more and API-X65 standard or higher. It is an object of the present invention to provide a hot-rolled steel sheet and a method for stably and inexpensively manufacturing the steel sheet.
• N 0 0 0 1 5 to 0 0 0 6
• N b 0.0 0 5 to 0.0 8%
• Low temperature toughness characterized in that the balance is Fe and inevitable impurities, and the elongation of the microstructure unit in the circumferential cross section of the pipe after pipe forming is 2 or less High-strength hot-rolled steel sheet for spiral pipes with excellent resistance.
• V 0.0 1% or more, 0.0 less than 4%
• a high-strength hot-rolled steel sheet for spiral pipes having excellent low-temperature toughness according to (1) or (2)
• the steel slab having the component according to any one of (1) to (3) is heated to a temperature that satisfies the SRT of the following formula at a temperature of 1 2 30 or less,
• Fig. 1 is a graph showing the relationship between the elongation of the microstructural unit and the ductile fracture surface ratio at ⁇ 20 in the DWT T test (the pipe circumferential direction hot coil width direction after pipe making).
• the microstructure was examined by grinding a cross section in the 45 ° direction in the rolling direction, using a sample cut from the 1 Z 4W or 3-4W position of the steel plate width as a representative value in the direction corresponding to the pipe circumferential direction after pipe forming. Etching with a Nital reagent and using a photo of the field of view at 1 Z 2 t of the plate thickness observed at a magnification of 200 to 500 times using an optical microscope.
• the elongation of the microstructure unit is JISG 0 5 5 1 — 2 0 0 5 steel.
• the crystal grain shape described in the microscopic test method for one crystal grain size is a linear test using a specimen in the longitudinal direction of the rolling.
• the thickness direction In accordance with the definition of the average length of crystal grains in the rolling direction obtained by the cutting method using a line divided by the average length of crystal grains perpendicular to the rolling direction, in the present invention, in the cross section in the 45 ° direction, the thickness direction The average length of crystal grains in the direction perpendicular to the thickness was divided by the average length of crystal grains parallel to the plate thickness direction.
• the microstructure unit mentioned here refers to the crystal grains of ferrite and second phase when the microstructure is a ferrite single phase or a microstructure containing ferrite ⁇ ⁇ ⁇ and a hard second phase.
• the grain boundary cannot be clearly identified by optical microscope observation as in w), it indicates a bucket.
• the former Mikuguchi tissue is a ferrite single phase or a microstructure containing Ferrite ⁇ and a hard second phase
• etching is performed using the above-mentioned Nital reagent, and the optical microscope is used as it is. Observed at magnification. That is, the microstructural unit is in this case the ferritic grains and Z or the second phase.
• the microstructure unit is a packet.
• a packet is a structural unit in which austenite grains are divided into many structural units with different orientations when undergoing the 7 ⁇ ⁇ transformation.
• Visualization of the bucket using EBSP- ⁇ I ⁇ ⁇ ⁇ because it is difficult to discriminate the bucket boundary by optical microscope observation with etching using Nital reagent
• EBSP—OIM TM Electro Back Scatter D iffraction P attern—Orientation Imae Microscopy
• SEM scanning electron microscope
• the system consists of a device and software that takes a Kikuchi pattern with a high-sensitivity camera and measures the crystal orientation of the irradiated spot in a short time by processing a combination image.
• the EBSP method enables quantitative analysis of the microstructure and crystal orientation of the bulk sample surface, and the analysis area is an area that can be observed with SEM. Depending on the resolution of the SEM, analysis can be performed with a minimum resolution of 20 nm.
• the analysis takes several hours and is performed by mapping tens of thousands of points in the grid at equal intervals in the region to be analyzed.
• the crystal orientation distribution and crystal grain size in the sample can be seen.
• the orientation difference of each packet is set to 15 °, and the packet is visualized from the mapped image to obtain the degree of expansion.
• Fig. 1 shows the relationship between the degree of elongation of the microstructure unit of the hot-rolled steel sheet and the ductile fracture surface ratio at 120 in the DWT T test for each rolling pattern.
• Fig. 1 shows the relationship between the degree of elongation of the microstructure unit of the hot-rolled steel sheet and the ductile fracture surface ratio at 120 in the DWT T test for each rolling pattern.
• the extension of the microstructure unit in the cross section in the 45 ° direction is 2 or less, but in the rolling patterns (2) and (3), the extension of the microstructural unit in the cross section in the 45 ° direction with respect to the rolling direction. It was newly discovered that the degree is over 2.
• the ductile fracture surface ratio at 120 in the DW TT test in which the extension of the microstructure unit in the cross section in the 45 ° direction with respect to the rolling direction is 2 or less and the width direction and the 45 ° direction in the rolling direction is 45.
• Ratio (the ratio of the ductile fracture surface ratio at 20 in the DWT T test in the direction of 45 ° in the rolling direction to the ductile fracture surface ratio at 120 in the DWT T test in the width direction) is 0.
• a value of 9 or higher was obtained, and the ductile fracture surface ratio in the pipe circumferential direction in the DWT T test of 2 O t: after forming into a spiral steel pipe was 70% or more, and it was strong as a spiral steel pipe for line pipe applications. -It has been found that the toughness balance satisfies the required properties.
• DWT T specimens for toughness evaluation are taken from the circumferential direction after pipe making.
• the spiral steel pipe does not match the circumferential direction after pipe making and the width direction of the hot coil that is the material. Therefore, when evaluating the toughness of the material hot coil, DWT T test specimens are collected and evaluated from the direction that coincides with the circumferential direction after pipe forming.
• the toughness of the material hot coil is It is known that the pipe deteriorates in the circumferential direction. Therefore, no matter how much the toughness in the width direction of the material hot coil is increased, it is not desirable for a spiral steel pipe if its deterioration margin in the pipe circumferential direction after pipe forming is large.
• the ductile fracture surface ratio at 120 in the DWTT test in the hot coil width direction is 85% or more, and the ductility in 120% in the DWTT test in the pipe circumferential direction after pipe forming in the rolling direction. If a ratio of fracture surface ratio of 0.9 or more is obtained, the pipe ductility in the DWTT test at 20 after pipe formation as a spiral pipe is sufficient, with a fracture surface ratio of 70% or more. A toughness value is obtained.
• the welding direction during spiral steel pipe making is determined by comprehensively determining the hot coil size, product steel pipe size, work efficiency, etc., but the best welding efficiency is in the 45 ° direction in the rolling direction. .
• the present invention was evaluated by adopting a 45 ° direction in the rolling direction as a representative value in the pipe circumferential direction after pipe making. However, it is not always necessary that the pipe making conditions of the spiral steel pipe be 45 °, and if necessary, evaluation may be made in the direction of the hot coil corresponding to the pipe circumferential direction after pipe making. Also, when evaluating, it is desirable to evaluate the direction of the hot coil corresponding to the circumferential direction of the pipe after pipe formation as much as possible, but in the range of ⁇ 5 ° around that direction. If it is evaluated, it may be considered within the scope of the present invention.
• the reason why the rolling pattern in the recrystallization temperature region affects the toughness difference between the hot coil width direction and the circumferential direction of the pipe after pipe forming has not been clarified.
• the strain introduced does not reach the strain required for recrystallization, and grain growth occurs predominantly, so that relatively coarse grains are elongated by rolling and the ⁇ ⁇ ⁇ transformation Later, the degree of expansion of the microstructural unit will increase.
• the reduction path in the recrystallization temperature region becomes larger than a certain reduction rate, dislocation introduction and recovery are continued during reduction, especially in the low temperature region at the later stage. Dislocation cell walls are formed by returning, and dynamic recrystallization that changes to sub-boundaries and large-angle boundaries occurs.
• C is an element necessary for obtaining the required strength. However, if it is less than 0.01%, the required strength cannot be obtained, and if it exceeds 0.1%, a large amount of carbide is formed as a starting point of fracture, and the toughness is deteriorated. In-situ weldability is significantly degraded. Therefore, the addition amount of C is set to 0.0 1% or more and 0.1% or less.
• Si Since Si has the effect of suppressing the precipitation of carbides that are the starting point of fracture, it is added in an amount of 0.05% or more, but if it exceeds 0.5%, the on-site weldability deteriorates. Furthermore, if it exceeds 0.15%, a tiger-stripe-like scale pattern may be generated and the appearance of the surface may be impaired. Therefore, the upper limit is desirably set to 0.15%.
• M n is a solid solution strengthening element. To obtain this effect, add 1% or more. However, even if Mn is added in excess of 2%, the effect is saturated, so the upper limit is made 2%. In addition, Mn promotes the center segregation of continuous forged steel slabs and forms a hard phase that is the starting point of fracture.
• P is an impurity and should be as low as possible. If it is contained in an amount of more than 0.03%, it will pray to the center of the continuous forged steel slab, causing grain boundary fracture and exhibiting low temperature toughness Therefore, it should be 0.03% or less. Furthermore, P has an adverse effect on pipemaking and on-site weldability, so considering these, it is desirable that P be less than 0.015%.
• O forms an oxide that becomes the starting point of fracture in steel and degrades brittle fracture and hydrogen-induced cracking. Furthermore, from the viewpoint of in-situ weldability, 0.002% or less is desirable.
• a 1 needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but the upper limit is set to 0.05% because it increases the cost. Further, if added too much, non-metallic inclusions are increased and low temperature toughness may be deteriorated, so the content is preferably 0.03% or less.
• N b is one of the most important elements in the present invention. Nb suppresses the recovery and recrystallization and grain growth of austenite ⁇ during and after rolling due to the dripping effect in the solid solution state and the pinning effect as Z or carbonitride precipitates, and in the crack propagation of brittle fracture It has the effect of reducing the effective crystal grain size and improving low temperature toughness.
• Nb has the effect of refining the microstructure after the transformation by delaying the Z ⁇ transformation and lowering the transformation temperature.
• addition of at least 0.05% or more is necessary. Desirably, it is 0.02 5% or more.
• more than 0.08% This effect not only saturates the effect, but also makes it difficult to form a solid solution in the heating process before hot rolling, forms coarse carbonitrides, and serves as a starting point for fracture. There is a risk of deterioration.
• T i is one of the most important elements in the present invention.
• Ti begins to precipitate as nitride at a high temperature immediately after solidification of the pieces obtained by continuous or ingot forming.
• This precipitate containing Ti nitride is stable at high temperatures, and does not completely dissolve even in subsequent slab reheating, but exhibits a pinning effect and coarsens austenite grains during slab reheating. Suppress and improve the low temperature toughness by refining the Miku mouth structure. It also has the effect of suppressing ferrite nucleation and promoting the formation of finely quenched structures in the key transformation. In order to obtain such an effect, it is necessary to add at least 0.05% Ti. On the other hand, even if added over 0.02%, the effect is saturated. Furthermore, when the amount of Ti added exceeds the stoichiometric composition with N (N—14 X 4 8 XT i ⁇ 0%), the precipitated Ti precipitates become coarse and the above effect cannot be obtained.
• N forms Ti nitride as described above, and suppresses coarsening of the austenite grains during slab reheating and improves low temperature toughness.
• the content is less than 0.0 0 1 5%, the effect cannot be obtained.
• the content exceeds 0.06%, ductility decreases due to aging, and formability during pipe forming decreases.
• N b—9 3/14 X (N-1 4/4 8 XT i) ⁇ 0.0 0 5% the fine Nb carbonization generated in the scraping process, which is a feature of the hot coil manufacturing process The amount of precipitates decreases and the strength decreases.
• Mo has the effect of improving hardenability and increasing strength. Also, Mo coexists with Nb to strongly suppress austenite recrystallization during controlled rolling, refine the austenite structure, and improve low-temperature toughness. There is an effect. However, even if added less than 0.01%, the effect is not obtained, and even if added over 0.1%, the effect is not only saturated but also ductility is lowered, and formability during pipe making is lowered. There is a concern.
• C r has the effect of increasing strength. However, even if added less than 0.01%, the effect cannot be obtained, and even if added over 0.3%, the effect is saturated. Also, there is a concern that if 0.2% or more is added, field weldability may be deteriorated, so less than 0.2% is desirable.
• Cu has the effect of increasing strength. It is also effective in improving corrosion resistance and hydrogen-induced cracking characteristics. However, the effect cannot be obtained even if added less than 0.01%, and the effect is saturated even if added over 0.3%. Also, if added at 0.2% or more, there is a concern that embrittlement cracks occur during hot rolling and cause surface defects, so less than 0.2% is desirable.
• the main purpose of adding these elements to the basic components is to increase the thickness of the plate that can be manufactured and to improve the properties such as the strength and toughness of the base material without impairing the excellent characteristics of the steel of the present invention. It is. Therefore, the amount of addition should be restricted by itself.
• V produces fine carbonitrides in the scraping process, which is a feature of the hot coil manufacturing process, and contributes to improving strength by precipitation strengthening.
• the effect cannot be obtained, and if added over 0.04% or more, the effect is not only saturated, but there is a concern that the field weldability may be reduced.
• Ni is less likely to form a hardened structure that is harmful to low-temperature toughness and sour resistance in the rolled structure (especially the center segregation zone of the slab) compared to Mn, Cr, and Mo.
• Toughness has the effect of improving strength without degrading on-site weldability. Even if added less than 0.01%, the effect cannot be obtained, and even if added over 0.3%, the effect is saturated. Also, Since Cu has the effect of preventing hot embrittlement, add 1 to 3 or more of Cu as a guide.
• B has the effect of improving hardenability and making it easier to obtain a continuously cooled transformation structure. Further, B enhances the hardenability improvement effect of Mo and has the effect of synergistically increasing the hardenability by coexisting with Nb. Therefore, add as necessary. However, if it is less than 0.0 0 0 2%, it is insufficient to obtain the effect, and if it exceeds 0 0 0 3%, slab cracking occurs.
• C a and R E M are elements that become the starting point of destruction and detoxify by changing the form of non-metallic inclusions that degrade sour resistance.
• addition of less than 0.005% has no effect, and if Ca is added over 0.05% and REM is added over 0.02%, a large amount of these oxides are formed. As a result, it is produced as coarse inclusions, which adversely affects the low-temperature toughness of the weld seam and on-site weldability.
• the steel containing these as the main components may contain Zr, Sn, Co, Zn, W, and Mg in total of 1% or less.
• Sn is brittle during hot rolling and may cause wrinkles, so 0.05% or less is desirable.
• microstructure of the steel sheet in the present invention will be described in detail.
• the degree of extension of the microstructure unit must be 2 or less.
• the continuous cooling transformation structure in the present invention is a microstructure containing one or more of ⁇ ° ⁇ , ⁇ ⁇ , q, ar and MA, and a small amount of ar and MA The total amount is 3% or less.
• Its microstructure is mainly Bainiticferrite ( ⁇ ° ⁇ ), Granularbainitcferrite (B ), Quasi — polygonalferrite (q), and is defined as a microstructure containing a small amount of residual austenite (rr) and artensite-austenite (A).
• rr residual austenite
• A artifact-austenite
• the production method preceding the hot rolling process by the converter is not particularly limited.
• it may be subjected to hot metal pretreatment such as hot metal dephosphorization and hot metal desulfurization, or refinement by a converter, or following the process of melting a cold iron source such as scrap in an electric furnace, etc.
• Ingredient adjustments are made so that the desired ingredient content is achieved by subsequent scouring,
• it may be produced by a method such as a thin slab forging in addition to a normal continuous forging or an ingot method.
• sour resistance specifications it is desirable to take measures against segregation such as unsolidified reduction in the continuous forging segment in order to reduce segregation of the slab center. Alternatively, it is effective to reduce the slab thickness.
• slabs obtained by continuous forging or thin slab forging it may be sent directly to a hot rolling mill with high temperature flakes, or after being cooled to room temperature and reheated in a heating furnace, hot rolled. Also good.
• slab direct feed rolling HCR: H0 TC harge Rolling
• the temperature is below this temperature, the coarse Nb carbonitride produced during slab production will not be sufficiently dissolved, and in the subsequent rolling process, the recovery of austenite wrinkles by Nb, suppression of recrystallization and grain growth, and Z Not only is the grain refinement effect due to the delay of ⁇ transformation not obtained, but it also has the effect of producing fine carbides in the scraping process, which is a feature of the hot coil manufacturing process, and improving the strength by precipitation strengthening. I can't get it. However, if the heating is less than 1 100, the amount of scale off is small and the inclusions on the surface of the slab may not be removed together with the scale by subsequent descaling, so the slab reheating temperature is 1 100 or more. Is desirable.
• the slab heating time of 1 2 OO t: or less is maintained for 20 minutes or more after reaching the temperature when it is necessary to sufficiently dissolve the Nb carbonitride.
• the subsequent hot rolling process usually consists of a rough rolling process consisting of several rolling mills including a reverse rolling mill and a finishing rolling process in which 6 to 7 rolling mills are arranged in tandem.
• the rough rolling process has the advantage that the number of passes and the amount of reduction in each pass can be set freely, but the time between passes is long, and recovery and recrystallization between passes may progress.
• the finish rolling process is a tandem type
• the number of passes is the same as the number of rolling mills, but the time between passes is short and it is easy to obtain a controlled rolling effect. Therefore, in order to realize excellent low temperature toughness, it is necessary to design a process that fully utilizes the characteristics of these rolling processes in addition to the steel components.
• the reduction path in the recrystallization temperature region is 25% or more, dislocation cell walls are formed by repeating the introduction and recovery of dislocations during reduction, particularly in the low temperature region at the later stage, and the sub- Dynamic recrystallization that changes to the grain boundary occurs, but it is short in a structure where grains with a high dislocation density and other grains are mixed, such as a microstructure mainly composed of dynamic recrystallization grains. Grain growth occurs over time, so it grows to relatively coarse grains before rolling in the non-recrystallized zone, and grains are formed by subsequent rolling in the non-recrystallized zone. There is a concern that the degree of extension will exceed 2 and the toughness deterioration in the pipe circumferential direction after pipe forming will increase. Therefore, the rolling reduction in each rolling pass in the recrystallization temperature range should be less than 25%.
• finish rolling Since the total rolling reduction in the non-recrystallization temperature range, which is a requirement of the present invention, cannot be satisfied by the process alone, it cannot satisfy the condition of 65% or more. Control rolling may be performed. In the case of the left, if necessary, it may wait until the temperature falls to the non-recrystallization temperature range, or cooling with a cooling device may be performed.
• a sheet bar may be joined between rough rolling and finish rolling, and finish rolling may be performed continuously.
• the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again to be joined.
• the total rolling reduction in the non-recrystallization temperature range is less than 65%, the controlled rolling effect cannot be obtained sufficiently and the low-temperature toughness deteriorates, so the total rolling reduction in the non-recrystallization temperature range should be 65% or more. .
• the plastic anisotropy of the steel sheet increases due to excessive crystal rotation due to rolling, and there is a concern that the difference in toughness depending on the direction of DWTT specimen collection will increase.
• the total rolling reduction in the temperature range is 80% or less.
• the finish rolling end temperature is not particularly limited, but it is desirable that the finish rolling end temperature is not less than the Ar 3 transformation point temperature.
• the finish rolling end temperature is finished at the Ar 3 transformation point temperature or more at the center of the plate thickness.
• the plate surface temperature is equal to or higher than the Ar 3 transformation point temperature.
• the rolling rate in the final stand is preferably less than 10% from the viewpoint of sheet shape accuracy.
• the Ar 3 transformation point temperature is simply expressed in relation to the steel composition by the following calculation formula, for example.
• n e Q M n + C r + C u + Mo + N i / 2 + 1 0 (
• Cooling will be performed after finishing rolling, but the time to start cooling should be within 5 seconds if necessary. If it takes more than 5 seconds from the end of finish rolling to the start of cooling, a large amount of polygonal ferrite will be contained in the microstructure ratio, and there is a concern that the strength will decrease.
• the cooling start temperature is not particularly limited, a large amount of polygonal ferrite is contained in the micro structure when cooling is started below the Ar 3 transformation point temperature. Since there is a concern about the decrease in strength, the cooling start temperature is preferably equal to or higher than the Ar 3 transformation point temperature.
• the cooling rate in the temperature range from the start of cooling to winding is 5: Z sec or higher.
• this cooling rate is 5 and less than 5 ec
• the precipitation of Nb carbonitride proceeds during cooling, and this precipitate inhibits the growth of a specific orientation with respect to the growth of austenite grains before transformation.
• the austenite grains are stretched and the influence after the a transformation is increased, and the degree of elongation of the Miku mouth tissue unit increases, and the toughness deterioration in the pipe circumferential direction after pipe formation increases. There are concerns.
• the cooling stop temperature and the coiling temperature are 5 0 0 and 6 0 0 and the temperature range is as follows. If the cooling is stopped at 600 or more and then rolled up, there is a concern that a large amount of coarse carbides unfavorable for low-temperature toughness may be generated, and coarse carbonitrides such as Nb are formed and become the starting point of fracture. Low temperature toughness may deteriorate sour resistance. On the other hand, when the cooling is finished at less than 500 and wound, fine carbonized precipitates such as Nb that are extremely effective for obtaining the desired strength cannot be obtained, and sufficient precipitation strengthening is obtained. I can't. Therefore, the cooling is stopped and the temperature range to be wound is 5 0 0 to 6 0 t. Example
• ⁇ Holding time '' is the holding time at the actual slab heating temperature
• ⁇ Recrystallization zone pass reduction ratio '' is the rolling reduction rate at each rolling pass in the recrystallization temperature zone
• ⁇ Cooling between passes '' refers to the presence or absence of cooling between rolling stands for the purpose of shortening the temperature waiting time that occurs between recrystallization temperature range rolling and non-recrystallization temperature range rolling.
• ⁇ FT '' is the finish rolling end temperature
• ⁇ A r 3 transformation point temperature '' is calculated A r 3 transformation point temperature
• ⁇ Time '' is the time from finish rolling to the start of cooling
• ⁇ Cooling rate '' is the average cooling rate when passing through the temperature range until winding of the cooling start temperature
• ⁇ CT '' Shows the tapping temperature.
• Table 5 shows the materials of the steel sheet thus obtained.
• the evaluation method is the same as described above.
• the “microstructure” is the microstructure of the steel sheet thickness at 1 Z 2 t
• the “stretchability of the microstructural unit” is the pipe circumferential cross-section after pipe formation at the center of the thickness
• the elongation defined as the average length of crystal grains perpendicular to the plate thickness direction divided by the average length of crystal grains parallel to the plate thickness direction, is the tensile test result.
• the result of the JIS No. 5 test piece is the “DWT T test” result.
• the “ductile fracture ratio” is the DWT T in each test temperature in the hot coil width direction and in the pipe circumferential direction after pipe forming.
• the ratio of the ductile fracture surface ratio is the ratio of the ductile fracture surface ratio in the DWT T test in the hot coil width direction—the pipe circumferential direction after pipe forming in the rolling direction relative to the ductile fracture surface ratio in 20 DW
• the ratio of ductile fracture surface ratio at 120 in the TT test is shown.
• steel Nos. 1, 2, 5, 6, 8, 9, 9, 15 and 16 are 8 steels, containing a predetermined amount of steel components, and after pipe forming in the rolling direction. It is characterized by a microstructural unit elongation of 2 or less in the cross section in the circumferential direction of the pipe.
• a material before spiral pipe forming it is equivalent to ⁇ 6 5 dales (tensile strength ⁇ 5 80 MPa, elongation ⁇ High strength hot-rolled steel sheet for spiral pipes with excellent low-temperature toughness with tensile strength that satisfies the specifications of 38%, pipe circumferential direction after pipe making—20% ductile fracture surface ratio ⁇ 70% Has been obtained.
• the steel component is outside the scope of claim 1 of the present invention, and sufficient low-temperature toughness after pipe formation is not obtained.
• Steel No. 19 has a steel component outside the scope of claim 1 of the present invention, so that sufficient low temperature toughness after pipe forming is not obtained.
• Steel No. 20 has a steel component outside the scope of claim 1 of the present invention, so that a sufficient tensile strength in the pipe circumferential direction after pipe forming cannot be obtained.
• Steel No. 2 1 has a steel component outside the scope of claim 1 of the present invention, so that sufficient ductility (elongation) in the pipe circumferential direction after pipe making cannot be obtained.
• Steel No. 19 has a steel component outside the scope of claim 1 of the present invention, so that sufficient low temperature toughness after pipe forming is not obtained.
• Steel No. 20 has a steel component outside the scope of claim 1 of the present invention, so that a sufficient tensile strength in the pipe circumferential direction after pipe forming cannot be obtained.
• Steel No. 2 1 has

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