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
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
  • B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous 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
  • 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
• • 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
  • C21D8/021—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 involving particular fabrication steps or treatments of ingots or slabs
• • 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
  • 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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
• • 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
  • C21D2201/00—Treatment for obtaining particular effects
  • C21D2201/05—Grain orientation
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to a high-strength hot-rolled mesh sheet for line pipes made of a hot coil having excellent low-temperature durability and a method for producing the same.
• steel pipes for line pipes can be classified according to their manufacturing processes as one-mill steel pipe, UOE steel pipe, electric steel pipe and spiral steel pipe, and-depending on their use and size, etc., except for seamless steel pipes.
• Each of them has a characteristic that it is commercialized as a steel pipe by forming a plate-shaped steel plate into a tubular shape and then seaming it by welding.
• these welded steel pipes can be divided according to whether hot coil or pre-coal is used as the material, the former being ERW and spiral steel pipes, and the latter being UOE steel pipes.
• the latter UOE steel pipe is generally used for high-strength, large-diameter, and thick-walled applications.
• the cost of the former UOE steel pipe is high, and the cost of the lead wire and the spiral steel pipe made of the hot coil are high.
• the demands for strength, large diameter, and thickening are increasing.
• the water quenching direct quenching method is a characteristic of the plate manufacturing process.
• I DQ 1 nterrupted D irect Quench
• strengthening of quenching is used to ensure strength. This is a special feature.
• the hot-rolled coil which is the material of ERW steel pipe and spiral steel pipe targeted by the present invention, has a winding process as a special feature of the process. take Therefore, it is impossible to stop the low-temperature cooling required for strengthening quenching. Therefore, it is difficult to ensure strength by strengthening quenching.
• the present invention has low-temperature toughness that can withstand use in cold regions.
• the severe unstable ductility required for gas line pipes is not only able to withstand use even in areas where destructibility is required.
• a hot rolled steel sheet for line pipe that has a thickness of 14 mm or more and a high strength of API-X70 standard or higher, but excellent absorption energy at the pipe operating temperature, and its steel sheet are specified at low cost. It is intended to provide a method that can be manufactured. Specifically, it is expected that the pipe will be sufficiently biased to meet the API-X70 standard after pipe formation, and the strength of the plate before pipe formation will be 6 20 MPa or more and An upper shelf energy in the DWTT test, which is an index of stable ductile fracture, and a pot plate having SATT (853 ⁇ 4> -20 or less in SATT (853 ⁇ 4> or less), and a method for stably and inexpensively manufacturing the plate The purpose is to provide
• the present invention is made by forming a continuous cooling transformation structure that is advantageous for low-temperature toughness and resistance to unstable fracture, instead of a ferrite structure, while being a very thick hot coil material.
• the means are as follows.
• the microstructure is a continuous cooling transformation structure, and the X 2 intensity ratio ⁇ 2 1 1 ⁇ / ⁇ 1 of the ⁇ 2 1 1 ⁇ surface parallel to the plate surface and the ⁇ 1 1 1 ⁇ surface in the texture at the center of the plate thickness 1 1 ⁇ is 1.1 or more, and the Nb and / or T 1 carbonitride precipitates have an intragranular precipitate density of 10 0 17 to 1 0 1 8 cm 3.
• Figure 1 shows the relationship between the surface strength ratio and S, I.
• Fig. 2 is a graph showing the relationship between the tensile strength and the precipitation density of Nb and / or T 1 carbonization precipitates precipitated in the grains.
• Figure 3 shows the relationship between tensile strength, microstructure, and temperature at which the ductile fracture surface ratio is 85% in the DWTT test.
• FIG. 4 is a graph showing the relationship between the cooling rate in the temperature range from the start of cooling to 700 and the surface strength ratio.
• FIG. S is a diagram showing the relationship between tensile strength, winding temperature, and heating temperature.
• Fig. 6 is a diagram showing the relationship between the time from the end of rolling to the beginning of cooling, the coiling temperature, and the outlet structure.
• BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first made the following: tensile strength and toughness of hot-rolled mesh plate (especially generation of separation and reduction in absorbed energy), and microstructure of pan plate As an example, the following experiment was performed assuming the case of the API I: X70 standard. 17 mm-thick test steel sheets prepared by melting the steel composition pieces shown in Table 1 under various hot rolling conditions were prepared, and the DWTT test results, separation index and reflection X-rays were prepared. The surface strength ratio was investigated. The survey method is shown below.
• DWTT Drop Weight Tear Test
• a strip-shaped test piece of 300 mmL X 75 mm WX plate thickness (t) mm was cut from the C direction, and a test piece with a 5 mm press notch was cut out.
• a top piece was prepared and carried out.
• a separation index (hereinafter referred to as S.I.) was measured in order to quantify the degree of separation that occurred on the fracture surface.
• S.I. is defined as the value obtained by dividing the total length of the separation parallel to the plate surface (xn xl i: 1 is the separation length) by the cross-sectional area (plate thickness X (75-notch depth)).
• the reflected X-ray surface intensity ratio (hereinafter referred to as the surface intensity ratio) is the ratio of the surface intensity of ⁇ 2 1 1 ⁇ to the surface intensity of ⁇ 1 1 1 ⁇ parallel to the plate surface at the center of the plate thickness, that is, This is the value defined as ⁇ 2 1 1 ⁇ / ⁇ 1 1 1 ⁇ and should be measured using X-rays in the manner shown in AS TM Standards Designation 8 1 — 6 3.
• the measurement device used in this experiment is a RINT 15500 type X-ray measurement device manufactured by Rigaku Corporation.
• Measurements were taken at a measurement speed of 40 minutes, using M o— ⁇ ⁇ as the X-ray source, tube voltage 60 kV, tube current 20 0 mA, and fill rate Z r — K 3 was used.
• a wide-angle goniometer is used with a step width of 0.0 ° 10 °, the slit is a divergence slit of 1 °, and the scattering slit is 1.
• the light receiving slit is 0.15 mm.
• the occurrence of separation is considered to be preferable for low temperature toughness by lowering the transition temperature.
• this should be improved.
• Fig. 1 shows the relationship between the surface strength ratio and SI of this hot-rolled steel sheet. 'When the surface strength ratio is 1.1 or higher, S.I, stabilizes at a low level, and becomes a value of 0.05 or lower.If the surface strength ratio is controlled to 1.1 or higher, the separation is suppressed to a level that does not cause any practical problems. It turns out that you can. More preferably, S.I, can be made 0.02 or less by controlling the surface intensity ratio to 1.2 or more.
• N b and / or T in the present invention were measured for the density of N b and Z or carbonitriding precipitates deposited in the mouth structure that is not grain boundaries.
• the intragranular precipitate density of the carbonitride precipitate of i is defined as the value obtained by dividing the number of carbonitride precipitates of Nb and / or Ti measured by the measurement method described later by the volume of the measurement range.
• a three-dimensional atom probe method was used to measure the density of Nb and / or Ti carbonitride precipitates precipitated in the grains.
• the measurement conditions are: the sample position temperature is approximately 70 K, the probe total voltage is 10 to 15 kV, and the pulse ratio is 25%. Each sample was measured three times for each grain boundary and within the grain, and the average value was used as the representative value.
• a sample cut from the 1Z4W or 3Z4W position of the steel plate width is polished into a cross section in the rolling direction, etched using a Nital reagent, and a magnification of 200 to 500 times using an optical microscope. This was done with a photograph of the field of view at 1 Z2 t of the plate thickness observed in.
• the volume fraction of the microstructure is defined by the area fraction in the above metal structure photograph.
• the continuous cooling transformation structure (Zw) is edited by the Japan Iron and Steel Institute Basic Research Group Paynite Research and Study Group; the latest report on the Paynay structure and transformation behavior of low-carbon steel Final Report (1 9 9 4 Japan Iron and Steel Institute)
• the microstructure including polygonal ferrite produced by a diffusive mechanism and martensite produced by a non-diffusion and shearing mechanism. It is a microstructure defined as a metamorphic structure in the middle stage.
• the continuous cooling transformation structure (Zw) is an optical microscope observation structure as described in the above references 1 2 5 to 1 27, and its mouth structure is mainly composed of B ainiticferr 1 te (a ° B), G ranularbainiti cferrite (a B) and Quasi — polyonalferrite (a Q), and is defined as a microstructure containing a small amount of residual austenite ⁇ (a r) and Martenste— austenite (A).
• the internal Q does not appear by etching as in the case of polygonal ferrite (PF), but the shape is uniform and clearly distinguished from PF.
• the perimeter of the target crystal grain Let lq be the diameter of the circle, and let d Q be the ratio (1 q no d Q) of the grains satisfying 1 Q d d ⁇ 3.5.
• the continuous cooling transformation structure (Zw) is defined as a mouth-mouthing structure containing ⁇ , ⁇ , ⁇ , r, MA, or one or more of them.
• MA has a total weight of 3% or less.
• Fig. 2 shows the relationship between the tensile strength of the hot-rolled steel sheet and the precipitate density of Nb and / or ⁇ ⁇ carbonitride precipitates precipitated in the grains.
• Z or T i precipitate density is 1 0 1 7 to 1 0 1 is eight Z cm 3 when most efficient precipitation strengthening effect of carbonitrides precipitate is obtained, the tensile strength is improved, the tensile strength After pipe making, it became clear that it would be 6 20 MPa or more with sufficient bias to fit the X70 grade range.
• the microstructure is a continuous cooling transformation structure, which is a requirement of the present invention
• the strength-toughness temperature at which the ductile fracture surface ratio in the DWT T test is 85%
• the tensile strength In order to achieve a tensile strength of 6 20 MPa or more and S ATT 85% of -20% or less, the tensile strength with sufficient bias to meet the X70 grade range after pipe formation. It is important that
• the mechanism by which the strength-elastic balance is improved by the continuous cooling transformation structure is not always clear, but the Mikuguchi organization is mainly B ainiticferrite ( ⁇ . ⁇ ), G ranular, bainiticferrite (a B). Quasi — It is composed of olygona 1 ferrite (q), has a relatively large boundary, and has a fine microstructure.It is considered to be the main influence factor of cleaving fracture propagation in brittle fracture. The effective crystal grain size is thought to be fine, and it is estimated that this led to improved toughness. These microstructures are special in that the effective crystal grain size is finer than that of the general Payne ⁇ produced by diffusive mash transformation.
• Figure 4 shows the relationship between the cooling rate and the surface strength ratio. A very strong correlation was observed between the cooling rate and the surface strength ratio, and it was found that the surface strength ratio was 1.1 or higher when the cooling rate was 15 Zsec or higher.
• the r ⁇ a transformation becomes shearing, and the Varian selection is proportional to the shear strain of the active slip ⁇ , which is considered to be the accumulation of ⁇ 2 1 1 ⁇ nodes / ND orientation. . Also, the crystallographic colony of ⁇ 2 1 1 ⁇ . Acts to relax the plastic anisotropy of ⁇ 1 1 1 ⁇ and the crystallographic colony of ⁇ 1 0 0 ⁇ and suppresses the generation of separation. It is estimated to be.
• Figure 5 shows the relationship between tensile strength, winding temperature and heating temperature.
• a very strong correlation was found between the coiling temperature and the tensile strength, and it became clear that the coiling temperature was 4 5 0 and above 6 5 0 and the tensile strength was equivalent to the X 70 grade.
• the precipitation density of Nb and Pino or TI carbonitride precipitates in the grains at a cutting temperature of 45 ° C. to 65 ° C. and below is within the scope of the present invention.
• 1 0 1 7 to 1 0 1 8 pieces were Zcm 3.
• the heating temperature is represented by the following formula:
• the precipitate density of the Nb and / or T 1 carbonitride precipitates precipitated in the grains when the solution temperature is less than the solution temperature calculated in ( 1) is within the range of the present invention. it was also found that not a 1 8 ⁇ cm 3.
• the hot coil which is the material of the torsional steel pipe and spiral steel pipe, which is the subject of the present invention, has a scraping process as a feature of the process, and scrapes thick materials at a low temperature due to restrictions on the facility capacity of the coiler. Is difficult is there. Therefore, precipitation strengthening is effectively used to ensure strength.
• precipitation strengthening elements such as Nb and T need to be solutionized in the slab heating process so that precipitation strengthening can be effectively manifested in the scraping process.
• the density of the precipitates is 10 17 to 10 L 8 cm 3 , which is the range of the present invention, and the strength is sufficiently secured.
• Fig. 6 shows the relationship between the time from the end of rolling to the start of cooling, the milling temperature, and the microstructure. It was found that a continuous cooling transformation structure, which is a requirement of the present invention, can be obtained when the time from the end of rolling to the start of cooling is within 5 seconds and the coiling temperature is 45 to 65 0 C.
• C is an element necessary for obtaining the required strength and microstructure. However, if it is less than 0.1%, the required strength cannot be obtained, and if it exceeds 0.1%, not only the carbide that becomes the starting point of fracture will be formed, but also the toughness will be deteriorated. The weldability is significantly deteriorated. Therefore, the addition amount of C is set to be 0.01% 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 smashing, it is added in an amount of 0.05% or more, but if over 0.5% is added, the field weldability deteriorates. Furthermore, if it exceeds 0.15%, it will be The upper limit is preferably set to 0.15% because the surface appearance may be damaged.
• Mn is a solid solution strengthening element.
• the austenite region temperature is increased to the low temperature side, and during the cooling after the end of rolling, there is an effect that it is easy to obtain a continuous cooling transformation structure which is one of the constituent requirements of the microstructure of the present invention.
• add 1% or more add 1% or more.
• the effect is saturated even if Mn is added in excess of 2%, so the upper limit is made 2%.
• 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 intergranular breakage and significantly reducing the low-temperature toughness. 3% or less. Furthermore, P has an adverse effect on pipemaking and on-site weldability, so considering these, 0 ⁇ 0 15% or less is desirable.
• S not only causes cracking during hot rolling, but if it is too much, the low temperature toughness deteriorates. Furthermore, S is bent near the center of the continuous forged steel slab, forming Mn S stretched after rolling, not only becoming the origin of hydrogen-induced cracking, but also generating pseudo-separation such as double sheet cracking. Concerned. Therefore, considering the sacrificial resistance, 0.001% or less is desirable.
• Oxide forms the starting point of fracture and deteriorates brittle fracture and hydrogen-induced cracking. Furthermore, from the viewpoint of on-site weldability, 0.002% or less is desirable.
• N b is one of the most important elements in the present invention. Nb suppresses the recovery, recrystallization and grain growth of austenite during and after rolling by the dripping effect in the solid solution state and the pinning effect as Z or carbonitride precipitates, and crack propagation of brittle fracture It has the effect of reducing the effective crystal grain size in and improving the low temperature toughness.
• ⁇ 1 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. These precipitates containing Ti nitride are stable at high temperatures, exhibit no pinning effect even during subsequent slab reheating, exhibit a pinning effect, and agglomerate austenite grains during slab reheating. Suppress and refine the microstructure to improve low temperature toughness
• N forms Ti nitride, suppresses coarsening of austenite grains during slab reheating, and has the effect of refining the effective grain size in subsequent controlled rolling, Low temperature toughness is improved by making the Miku mouth structure a continuous cooling transformation structure.
• 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.
• the main purpose of adding these elements to the basic components is to increase the manufacturable plate thickness and improve properties such as the strength and toughness of the base material without impairing the excellent characteristics of the steel of the present invention. is there. Therefore, the amount added should be restricted by itself.
• V produces fine carbonitrides in the cutting process, which is a feature of the hot coil manufacturing process, and contributes to improving strength by precipitation strengthening.
• the effect cannot be obtained by adding less than 0.01%, and the effect is saturated even if added over 0.3%.
• 0.04% or more is added, there is a concern that the on-site weldability may be reduced, so less than 0.04% is desirable.
• Mo has the effect of improving hardenability and increasing strength.
• Mo coexists with Nb and has the effect of strongly suppressing the recrystallization of austenite during controlled rolling, making the austenite structure finer, and improving low-temperature toughness.
• the effect cannot be obtained even if less than 0.01% is added, and the effect is saturated even if added over 0.3%.
• 0.1 If added over 5% the ductility deteriorates, and there is a concern that the formability during pipe forming may be reduced, so less than 0.1% is desirable.
• C r has the effect of increasing strength. However, the effect cannot be obtained even if less than 0, 0 1% is added, and the effect is saturated even if added over 0.3%. Also, if adding more than 0.2%, there is a concern that the on-site weldability may be reduced, so less than 0.2% is desirable.
• Cu is effective in improving corrosion resistance and * element-induced cracking resistance.
• the effect cannot be obtained even if less than 0.1% is added, 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 cracking may occur during hot rolling and cause surface flaws. .
• Ni is less likely to form a hardened structure that is harmful to low temperature resistance and heat resistance in the rolled structure (especially the center segregation zone of the slab) compared to Mn, Cr, and Mo.
• Low temperature toughness has the effect of improving strength without degrading the local 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, 1/3 or more of the Cu content is added for reference.
• B has the effect of improving hardenability and making it easier to obtain a continuously cooled transformation structure. Furthermore, B enhances the hardenability of Mo and also has the effect of synergistically increasing hardenability in coexistence with Nb. Therefore, add as necessary. However, if it is less than 0.0 0 02%, it is insufficient to obtain the effect, and if added over 0.03%, slab cracking occurs.
• C a and R EM are elements that are detrimental by changing the form of non-metallic inclusions that become the starting point of destruction and degrade sour resistance. However, even if added less than 0.000%, there is no effect. Addition of more than 0 0,5% or 0, 0,2% for REM produces a large amount of these oxides, forming clusters and rough inclusions, resulting in low-temperature toughness degradation of weld seams and on-site weldability Also has an adverse effect.
• the steel containing these as the main components may contain Zr, Sn, Co, Zn, W, and Mg in total of 1% or less. However, since Sn may become brittle during hot rolling and generate wrinkles, it is preferably 0.05% or less.
• the microstructure of the steel sheet in the present invention will be described in detail.
• the microstructure is a continuous cooling transformation structure, and the intragranular precipitate density of the carbonitride precipitates of Z and T i is from 10 17 to 10 1 8 in it is necessary e where a number cm 3, and the present invention definitive continuously cooled transformed structure (Z w), ⁇ ° ⁇ , ⁇ , aq, rr.
• MA one or Miku port comprising two or more It is an organization, and a small amount of rr and MA makes the total amount 3% or less.
• the production method preceding the hot rolling process using a converter is not particularly limited.
• scouring with a converter through hot metal pretreatment such as hot metal dephosphorization and hot metal desulfurization, or various processes following the process of melting cold iron iron such as scrap in an electric furnace, etc.
• the components may be adjusted so that the desired component content is obtained by secondary scouring, and then forged by a method such as thin slab forging in addition to normal continuous forging and forging by ingot method.
• sour-resistant specifications it is desirable to take measures against segregation such as unsolidified reduction in a small continuous segment to reduce segregation of the center of the slab. It is also effective to reduce the slab thickness.
• a slab obtained by continuous or thin slab fabrication it may be sent directly to a hot rolling mill as it is at high temperature, or after being cooled to room temperature and reheated in a heating furnace, hot rolled. May be.
• slab direct rolling H ⁇ TC harge Rolling
• the austenite transformation is applied to reduce the austenite grain size during reheating of the slab due to the transformation from a to a.
• the slab reheating temperature (S RT) is
• the temperature calculated in. If the temperature is lower than this temperature, the coarse Nb carbonitride produced during slab production will not dissolve sufficiently, and in the subsequent rolling process, the recovery of austenite wrinkles by Nb will be suppressed. As long as the effect of grain refinement due to the delay in the process cannot be obtained, fine carbides are generated in the scraping process, which is a feature of the hot coil manufacturing process, and the effect of improving the strength by precipitation strengthening is obtained. Absent. However, if the heating is less than 1 100, the amount of scale-off is so small that the inclusions on the surface of the slab cannot be removed together with the scale by subsequent descaling, so the slab reheating temperature is 1 1 0 0 or more. I want it.
• the grain size of austenite becomes coarse, and the effect of refining the effective crystal grain size in the subsequent controlled rolling cannot be obtained.
• the effect of improving the low-temperature inertia by the continuously cooled transformation structure cannot be enjoyed. More preferably, it is 1 2 0 0 or less.
• the slab heating time allows the Nb carbonitride to sufficiently dissolve To maintain the temperature, hold it for at least 20 minutes.
• the 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 6-7 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 there is a risk of recovery and recrystallization between passes.
• the finishing shoring 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.
• controlled rolling in the non-recrystallization temperature range may be performed after the rough rolling process.
• time may be taken until the temperature falls to the non-recrystallization temperature range, or cooling with a cooling device may be performed.
• a sheet roll may be joined to open rough rolling and finish rolling, and finish rolling may be performed continuously.
• the assembly bar may be rolled in a coil shape, and stored in a cover having a heat retaining function as necessary, and then rolled back again before joining.
• the finish rolling process rolling is performed in the non-recrystallization temperature range, but if the temperature at the end of rough rolling does not reach the non-recrystallization degree range, the temperature falls to the non-recrystallization temperature range as necessary. It is possible to wait until the time is reached or to cool with a cooling device between the rough finish rolling stands if necessary. If the total rolling reduction in the non-recrystallization temperature range is less than 65%, the effect of refining the effective crystal grain size by controlled rolling cannot be obtained, and the microstructure does not become a continuous gun cooling transformation weave. Since toughness deteriorates, the total rolling reduction in the non-recrystallization temperature region should be 65% or more.
• the total rolling reduction in the non-recrystallization temperature region is desirably 70 3 ⁇ 4 or more.
• the finish rolling finish temperature ends at or above the A r 3 transformation point temperature.
• the temperature is below the Ar 3 transformation temperature at the center of the plate thickness, ⁇ + a two-phase region rolling occurs, and significant separation occurs on the ductile fracture surface, resulting in a significant decrease in absorption energy.
• the finish rolling finish temperature ends at or above the Ar 3 transformation point temperature in the center of the plate thickness.
• it is desirable that the plate surface temperature is not less than the Ar 3 transformation point temperature.
• the rolling rate in the final stand is preferably less than 10% from the viewpoint of sheet metal accuracy.
• Mn e q Mn + C r + C u + Mo + N l Z 2 + 1 0 (N b-0. 0 2).
• the cooling start temperature is not particularly limited, but is less than the A r 3 transformation point temperature.
• 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 700 is set to 15 5 sec or more.
• the effect of the present invention can be obtained without any particular limitation on the upper limit of the cooling rate.For example, even if a cooling rate exceeding 50 Zsec is achieved, the effect is not only saturated, but also Since there is concern about plate warping due to thermal strain, it is desirable to set it to 50 "C / sec or less.
• the cooling rate in the temperature range from 700 to coiling is not particularly limited with respect to the suppression of separation generation, which is an effect of the present invention, so air cooling or an equivalent cooling rate may be used.
• air cooling or an equivalent cooling rate may be used in order to suppress the formation of coarse carbides and to obtain a further excellent strength-toughness balance.
• the average cooling rate from the end of rolling to winding is 15 / s e or more.
• Cooling stop temperature and coiling temperature should be between 45 0 and 65 0 and the following temperature range. Cooling is stopped at 5 50 or more, and if it is wound up after that, it contains coarse carbide such as pearlite, which is undesirable for low temperature toughness. And a mimic mouth structure of a continuously cooled transformation structure, which is a requirement of the present invention, cannot be obtained. In addition, Nb and other large carbonitrides are formed, which becomes the starting point of fracture, and low temperature toughness may deteriorate sour resistance.
• the steels A to J having the chemical components shown in Table 2 are melted in a converter, directly fed or reheated after continuous forging, and rolled down to 20 mm in the final rolling following rough rolling to a sheet thickness of 4 mm. They were wound up after cooling in a run-out table. However, the indication about the chemical composition in the table is mass%. Details of the manufacturing conditions are shown in Table 3. Here, “component” is the symbol for each slab piece shown in Table 2, “heating temperature” is the actual slab heating temperature, and “solution temperature” is
• the “holding time” is the holding time at the actual slab ripening temperature, and “inter-pass cooling” is performed for the purpose of shortening the temperature waiting time that occurs before rolling in the non-recrystallization temperature range. Whether there is cooling between rolling stands
• Unrecrystallized zone total rolling reduction is the total rolling reduction rate of rolling performed in the non-recrystallization temperature range
• “FT” etc. is the finish rolling finish temperature
• “A r 3 transformation” “Point temperature” is calculated Ar 3 Transformation point temperature
• “Time to start cooling” is the time from finish rolling to start cooling
• “Cooling rate J up to 700” is cooling start The average cooling rate when passing through the temperature range from 700 to 700
• “CT” indicates the scraping temperature.
• Table 4 shows the materials of the steel sheet thus obtained.
• the evaluation method is the same as described above.
• the “microstructure” is the microstructure at 1 to 2 t of the steel plate thickness
• the “surface strength ratio” is ⁇ 2 1 1 ⁇ parallel to the plate surface in the collective weaving at the center of the plate thickness.
• the reflection X-ray intensity ratio ⁇ 2 1 1 ⁇ / ⁇ 1 1 1 ⁇ between the surface and the ⁇ 1 1 1 ⁇ surface is defined as “precipitate density”.
• steel Nos. 1, 2, 3, 1 1, 1 2, 1 3, 1 4, 1 5, 1 6, 1 8, 24, 2 5, 2 7, 2 8 It contains a predetermined amount of steel component, its microstructure is a continuous cooling transformation structure, and the surface strength ratio parallel to the plate surface is 1.1 or more in the texture at the center of the plate thickness.
• a high-strength hot-rolled steel sheet for line pipes with excellent low-temperature toughness having tensile strength equivalent to X70 grade as a material before pipe making is obtained.
• steels other than the above are outside the scope of the present invention for the following reasons.
• steel No. 4 has a heating temperature outside the scope of claim 6 of the present invention, and therefore, the intragranular precipitation density of the target precipitate according to claim 1 cannot be obtained. Not enough tensile strength is obtained.
• Steel No. 5 has a heating retention time outside the range of claim 6 of the present invention, and therefore, the target intragranular precipitate density of claim 1 cannot be obtained, and sufficient tensile strength is obtained. Absent.
• the total reduction ratio in the non-recrystallization temperature region is outside the range of claim 6 of the present invention, so the target microstructure of claim 1 is not obtained, and sufficient low temperature toughness is obtained. Absent. In Steel No.
• the heating temperature is outside the range of Claim 6 of the present invention, so the target microstructure of Claim 1 cannot be obtained, and sufficient low temperature toughness is not obtained.
• the target Miku ⁇ structure according to claim 1 cannot be obtained, and sufficient low temperature toughness cannot be obtained.
• No. 9 has a cooling rate outside the range of claim 6 of the present invention, so the desired surface strength ratio according to claim 1 cannot be obtained, and sufficient low-temperature toughness has not been obtained.
• the manufacturing method of the present invention makes it possible to obtain a large quantity of hot coils for ERW steel pipes and spiral steel pipes at low cost. It can be said that there is.

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