WO2018117575A1 - Plaque d'acier laminée à chaud de résistance élevée ayant un faible écart de matière et une excellente qualité de surface et son procédé de fabrication - Google Patents

Plaque d'acier laminée à chaud de résistance élevée ayant un faible écart de matière et une excellente qualité de surface et son procédé de fabrication Download PDF

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WO2018117575A1
WO2018117575A1 PCT/KR2017/014963 KR2017014963W WO2018117575A1 WO 2018117575 A1 WO2018117575 A1 WO 2018117575A1 KR 2017014963 W KR2017014963 W KR 2017014963W WO 2018117575 A1 WO2018117575 A1 WO 2018117575A1
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steel sheet
rolled steel
hot rolled
surface quality
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PCT/KR2017/014963
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English (en)
Korean (ko)
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공종판
정제숙
박교선
박경미
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주식회사 포스코
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Priority to CN201780080252.1A priority Critical patent/CN110100033B/zh
Publication of WO2018117575A1 publication Critical patent/WO2018117575A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • 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
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C21D8/0294—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a localised treatment
    • 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
    • C22C38/06—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high strength hot rolled steel sheet having a low material deviation and excellent surface quality, and a method of manufacturing the same.
  • the so-called DP (Dual Phase) steel is mainly composed of two phases of ferrite and martensite, and is one of the representative steel grades having low yield strength.
  • Patent Documents 1 and 2 Patent documents related to the manufacturing technology of such high strength hot rolled DP steel are Patent Documents 1 and 2, but these are all related to the method of manufacturing in the existing mill process, the problem that the material deviation in the production in the actual line largely in the width and length direction It is difficult to avoid.
  • the final finish rolling speed is usually 400mpm or more, so the target material must be wound due to the manufacturing characteristics of DP steel, which must be wound at a temperature lower than Ms (martensite transformation start temperature). There is a problem that it is not easy to secure stably.
  • Ms martensite transformation start temperature
  • a new steel manufacturing process which is recently attracting attention, is a process for manufacturing a so-called thin slab (mini-mill process), which has a small temperature deviation in the width and length direction of the strip due to the characteristics of the process. It is attracting attention as a process with potential.
  • Patent document 3 relates to a method for producing tensile strength 590 MPa class hot rolled DP steel in a batch method in a mini mill process, and limits the final steel sheet thickness to 3.0 mm.
  • the reason for this is that in the case of the existing mini mill process, the bar plate is wound in a coil box and pulled out so that this process is required every time a steel sheet is produced. And it is difficult to produce hot rolled coil (Coil) having a thickness of less than 3.0mm due to the poor flowability and very high risk of sheet breakage.
  • Patent Document 1 United States Patent Application Publication No. 4285741
  • Patent Document 2 U.S. Patent Publication No.4325751
  • Patent Document 3 Korean Unexamined Patent Publication No. 10-2012-0052022
  • One aspect of the present invention provides a high-strength hot-rolled steel sheet of 780MPa grade and its tensile strength which is excellent in surface quality and at the same time significantly reduced the width, longitudinal material deviation of the steel sheet using the continuous continuous rolling mode in the play-rolling direct connection process To provide.
  • One aspect of the present invention is by weight, C: 0.03-0.065%, Mn: 1.5-2.5%, Si: 0.1-0.5%, P: 0.01-0.05%, S: 0.01% or less, Cr: 0.1-0.6% Al: 0.05% or less, Ti: 0.01% to 0.05%, N: 0.001% to 0.010%, remaining Fe and other unavoidable impurities,
  • the microstructure is a high-strength hot rolled steel sheet having a low surface area deviation and excellent surface quality, including ferrite 50 to 70%, martensite 20 to 40%, and bainite 5 to 15%.
  • another aspect of the present invention is by weight, C: 0.03-0.065%, Mn: 1.5-2.5%, Si: 0.1-0.5%, P: 0.01-0.05%, S: 0.01% or less, Cr: 0.1 Continuous casting of molten steel containing -0.6%, Al: 0.05% or less, Ti: 0.01-0.05%, N: 0.001-0.010%, remaining Fe and other unavoidable impurities with a thin slab having a thickness of 60-120 mm;
  • Removing the scale by sequentially passing the bar plate through two rows of spraying cooling water at a pressure of 100 to 250 bar and one row of spraying cooling water at a pressure of 50 to 150 bar;
  • the continuous continuous rolling mode in the play-rolling direct connection process not only the surface quality is excellent, but also the material deviation in the width and length direction of the steel sheet is remarkably reduced, and the tensile error is excellent and the thickness is 3.0 mm or less.
  • Strength 780MPa class high strength hot rolled steel sheet can be manufactured.
  • the hot rolled steel sheet produced by the present invention is a thin material (3.0 mm or less in thickness) and the edge portion and the surface scale quality is good, it is possible to manufacture a high-quality PO material in the general hot-rolling pickling process, hot rolled material (more than 3.0 mm thick) It is differentiated from the existing mini mill process that can only produce, and it is excellent in price competition and can greatly improve added value.
  • the thin slab playing method can eliminate the reheating process in the existing mill, which can save energy and improve productivity, and can use the steel dissolved in scrap such as scrap in the electric furnace to improve the recycling of resources. Can be.
  • FIG. 2 is a graph showing the ferrite grain size distribution of Inventive Example 2 measured using an EBSD (Electron BackScatter Diffraction).
  • EBSD Electro BackScatter Diffraction
  • FIG. 3 is a photograph of the surface of the PO material of Inventive Example 2.
  • FIG. 4 is a schematic diagram of a process using a continuous continuous rolling mode in the play-rolling direct connection process.
  • the bar plate is wound on a coil box and pulled out so that this process is required every time a steel sheet is produced in a batch method. And it is not good mailing ability, the risk of plate breakage is very high, it is difficult to produce a hot rolled coil (Coil) of less than 3.0mm thick, and deeply studied to solve this problem.
  • Coil hot rolled coil
  • the surface quality is excellent by using the continuous rolling mode in the play-rolling direct connection process, and at the same time, the thickness of the material in the width and length direction of the steel sheet is significantly reduced to 3.0. It was confirmed that a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or less with a tensile strength of mm or less could be produced, and thus, the present invention was completed.
  • High-strength hot-rolled steel sheet having a low material deviation and excellent surface quality is a weight%, C: 0.03 to 0.065%, Mn: 1.5 to 2.5%, Si: 0.1 to 0.5%, P: 0.01 to 0.05% , S: 0.01% or less, Cr: 0.1 ⁇ 0.6%, Al: 0.05% or less, Ti: 0.01 ⁇ 0.05%, N: 0.001 ⁇ 0.010%, remaining Fe and other unavoidable impurities, and the microstructure is in the area fraction Ferrites 50-70%, martensite 20-40% and bainite 5-15%.
  • the unit of each element content is weight%.
  • Carbon (C) is an important element to increase the strength of the steel sheet and to secure a composite structure composed of ferrite and martensite.
  • C content When the C content is less than 0.03%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the C content is more than 0.065%, molten steel can be leaked because the alloy steel is manufactured by high-speed playing, and a solidification cell having a non-uniform thickness may be formed, which may cause an operation accident. Therefore, it is preferable that C content is 0.03 to 0.065%.
  • the lower limit of the C content may be 0.035%, and the lower limit may be 0.04%.
  • the more preferable upper limit of C content may be 0.06%.
  • Manganese (Mn) is an element that has a very high effect of solid solution strengthening and promotes the formation of a complex structure composed of ferrite and martensite.
  • the Mn content is less than 1.5%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the Mn content is more than 2.5%, weldability, hot rolling property, and the like may be inferior. In addition, excessive addition of Mn content reduces the delta-ferrite region at the temperature near the solidification, so that the apolytic reaction can occur even in the low C region. Molten steel spills can cause operational accidents. Therefore, the Mn content is preferably 1.5 to 2.5%.
  • the lower limit of the Mn content may be 1.6%, and the lower limit may be 1.7%.
  • the more preferable upper limit of Mn content may be 2.4%, and a still more preferable upper limit may be 2.3%.
  • Silicon (Si) is a useful element that can secure the ductility of the steel sheet. It is also an element that promotes ferrite formation and promotes martensite formation by encouraging C concentration into unmodified austenite.
  • Si content is less than 0.1%, it is difficult to secure the above-described effects sufficiently.
  • Si content is more than 0.5%, red scale is generated on the surface of the steel sheet, and traces may remain on the surface of the steel sheet after pickling, thereby degrading the surface quality. Therefore, it is preferable that Si content is 0.1 to 0.5%.
  • the lower limit of Si content may be 0.12%, and the upper limit may be 0.4%.
  • Phosphorus (P) is an element showing the effect of strengthening the steel sheet.
  • the content of P is preferably limited to 0.01 to 0.05%.
  • S Sulfur as impurities may segregate during MnS non-metallic inclusions and performance solidification in steel, causing hot cracks.
  • the content should be controlled as low as possible, preferably at 0.01% or less.
  • the lower limit of the S content does not need to be particularly limited, but excluding the S content to 0% may be excessive, so 0% may be excluded.
  • Chromium (Cr) is an element that improves hardenability and increases the strength of steel.
  • Cr content When the Cr content is less than 0.1%, the above effects are insufficient. On the other hand, when the Cr content is more than 0.6%, there is a problem that the ductility of the steel sheet is lowered. Therefore, it is preferable that Cr content is 0.1 to 0.6%.
  • Aluminum (Al) may be concentrated on the surface of the steel sheet to degrade the plating property, while suppressing carbide formation to increase the ductility of the steel.
  • the reheating process in the existing hot-dense mill can improve the energy saving and productivity, but the temperature of the slab surface or the edge portion may drop due to the cold cooling of the slab surface. This can result in excessive precipitation of AlN, resulting in inferior edge quality of the cast and / or bar plates due to high temperature ductility degradation.
  • the content should be controlled as low as possible, preferably at 0.05% or less.
  • the lower limit of the Al content does not need to be particularly limited, but excluding the Al content of 0% may be excessive, so 0% may be excluded.
  • Titanium (Ti) is an element that increases the strength of steel as a precipitate and nitride forming element.
  • Ti is an element that reduces the amount of AlN precipitates by removing the solid solution N through the formation of TiN near the solidification temperature, thereby preventing the reduction of high-temperature ductility. Therefore, precise control is necessary because Ti is a very useful element to solve the surface and / or edge quality problems and strength obtained in thin slab high-speed performance.
  • the Ti content is less than 0.01%, the above effects are insufficient.
  • the Ti content is more than 0.05%, it is possible to increase the manufacturing cost and lower the ductility of the ferrite. Therefore, the Ti content is preferably 0.01 to 0.05%.
  • Nitrogen (N) is an austenite stabilizing and nitride forming element.
  • the austenite stabilization effect is insufficient.
  • the N content is more than 0.010%, it increases the precipitation strengthening effect by reacting with the precipitate-forming element, but may cause a sharp drop in ductility.
  • the surface and edge quality may be inferior due to excessive precipitation such as AlN due to the cooling of the slab in thin slab high speed performance.
  • N content is preferably 0.001 to 0.010%.
  • the remaining component of the present invention is iron (Fe).
  • impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.
  • one or more of Cu, Ni, Mo, Sn, and Pb may be included, and the sum thereof may be 0.2 wt% or less.
  • the tramp element is an impurity element derived from scrap used as a raw material in the steelmaking process, and when the sum is more than 0.2%, the surface crack of the thin slab and the surface quality of the hot rolled steel sheet may be degraded.
  • Ceq represented by the following formula (1) may be 0.14 ⁇ 0.24.
  • each element symbol is a value representing each element content in weight%.
  • Equation (1) is a component relational formula for securing weldability of the steel sheet.
  • Ceq value 0.14 to 0.24
  • excellent spot weldability can be ensured and excellent mechanical properties can be given to the welded portion. .
  • Ceq is less than 0.14, there is a problem that it is difficult to secure the target tensile strength due to the low hardenability. On the other hand, if Ceq is greater than 0.24, the weldability may deteriorate and the physical properties of the weld portion may deteriorate.
  • the microstructure of the hot rolled steel sheet according to the present invention includes 50 to 70% of ferrite, 20 to 40% of martensite, and 5 to 15% of bainite.
  • the ferrite fraction of the microstructure of the hot rolled steel sheet according to the present invention is more than 70%, it is difficult to secure the target strength, and if less than 50%, the fraction of the remaining martensite and bainite structure is increased to secure ductility. There is difficulty.
  • the martensite fraction is more than 40%, the strength is too high to secure ductility, and less than 20% may be difficult to secure the target strength.
  • the reason for including a part of bainite in the microstructure of the hot-rolled steel sheet is as follows.
  • bainite area fraction is less than 5%, the above effects are insufficient. On the other hand, if the content is greater than 15%, the strength may be too high to secure ductility.
  • the ferrite grains may have an average size of 5 ⁇ m or less as measured by a circle equivalent diameter.
  • the size of the ferrite grains exceeds 5 ⁇ m it may be difficult to secure the target strength and processability. More preferably, it may be 3 micrometers or less.
  • the grain size exceeds about 5 ⁇ m because the finish rolling temperature is finish-rolled at a temperature around 900 ° C., which is much higher than the normal Ar3 temperature (810 to 850 ° C.) to secure the mail flow during finish rolling. Has organization.
  • the play-roll direct connection process according to the present invention it is possible to secure and control uniform temperature by isothermal and isothermal rolling due to the characteristics of the process, thereby securing the final structure uniformly, and to finish rolling at a lower temperature than Ar3. Fine control
  • the thickness of the hot rolled steel sheet of the present invention may be 3.0mm or less. This is because, unlike the conventional mini mill process, which can produce only hot rolled material (more than 3.0 mm in thickness), the hot rolled steel sheet can be produced in a thickness of 3.0 mm or less according to the manufacturing method of the present invention. More preferably, the thickness may be 2.0 mm or less.
  • the hot rolled steel sheet of the present invention may have a tensile strength of 780 MPa or more, an elongation of 15% or more, and a material deviation of the tensile strength of 15 MPa or less.
  • a method of manufacturing a high strength hot rolled steel sheet having a low material deviation and excellent surface quality comprising: continuously casting molten steel having the above-described alloy composition into a thin slab having a thickness of 60 to 120 mm; Spraying coolant at a pressure of 150 bar or more to the thin slab to remove scale; Roughly rolling the scaled thin slab so that the edge portion temperature of the bar plate is about 850 to 1000 ° C.
  • the new steel manufacturing process which is attracting attention recently, is a process of using the so-called thin slab (mini mill process), which is a direct rolling-rolling process, and has a good material deviation due to the small temperature deviation in the width and length directions of the strip. It is attracting attention as a process with the potential to manufacture tissue steel.
  • the coil In case of batch mode, in order to compensate for the difference between the playing speed and rolling speed, the coil is rolled up in the coil box before the finishing mill and finish rolling is performed. Problems such as breakage may occur.
  • Figure 4 shows an example of a process using the continuous continuous rolling mode in the play-rolling direct connection process.
  • a thin slab (a) having a thickness of 50 to 150 mm is manufactured, and there is no coil box between the roughing mill 400 and the finishing mill 600, so that the steel sheet can be continuously rolled to obtain a good flowability.
  • the risk of plate breakage is very low, making it possible to produce thin films with a thickness of less than 3.0mm.
  • Rough mill scale breaker (RSB) 300 in front of rough mill 400 and finishing mill scale breaker 500 in front of finishing mill 600 (FSB) for easy surface scale removal.
  • isothermal isothermal rolling is possible with a difference in rolling speed of 5% or less in one steel sheet, so that the steel plate width and longitudinal temperature variation are remarkably low and precisely cooled in the run out table (ROT).
  • the controllable steel sheet can be manufactured with excellent material variation.
  • the molten steel having the alloy composition described above is continuously cast into a thin slab having a thickness of 60 to 120 mm.
  • the thickness of the thin slab exceeds 120mm, not only the high speed casting is difficult, but also the rolling load increases during rough rolling, and when the thickness of the slab is less than 60mm, a temperature drop of the cast slab rapidly occurs to form a uniform structure.
  • the casting speed of the continuous casting may be 4 ⁇ 8mpm.
  • the casting speed of the continuous casting is preferably 4 ⁇ 8mpm.
  • Cooling water is sprayed to the heated thin slab at a pressure of 150 bar or more to remove scale.
  • the surface scale thickness may be removed to 300 ⁇ m or less by spraying a cooling water of 40 ° C. or less at a pressure of 150 bar or more from a rough mill scale breaker (RSB) nozzle.
  • RTB rough mill scale breaker
  • the thin slab from which the scale is removed is roughly rolled on the rough rolling side so that the edge temperature of the bar plate is 850 to 1000 ° C. to obtain a bar plate.
  • it can be rough rolling in a roughing mill consisting of 2 to 5 stands.
  • edge portion temperature When the edge portion temperature is less than 850 ° C., a large amount of AlN precipitates are generated, and thus there is a problem in that edge crack generation sensitivity is very high due to the decrease in high temperature ductility.
  • edge temperature if the edge temperature is more than 1000 °C the temperature of the thin slab core is too high, there is a possibility that a large number of arithmetic scale occurs, the surface quality is inferior after pickling.
  • the rough rolling may be performed so that the surface temperature of the thin slab is 1000 ⁇ 1200 °C at the entrance side of the rough rolling.
  • the surface temperature of the thin slab at the inlet side of the roughing mill is less than 1000 ° C., there is a possibility that cracks may occur at the edge of the bar plate during the rough rolling load increase and the rough rolling process, and this may cause an edge defect of the hot rolled steel sheet. If the slab surface temperature exceeds 1200 ° C., problems such as deterioration of the hot rolled surface due to the remaining hot rolled scale may occur.
  • the rough rolling can be performed so that the cumulative reduction rate is 60 ⁇ 90%.
  • the higher the rolling reduction rate during rough rolling the more uniform the microscopic distribution of Mn, Si, Cr, etc., which are important elements for the production of high strength steel, and the smaller the temperature gradient in the width and thickness direction of the strip. Do.
  • the cumulative reduction ratio is less than 60%, the above-described effects are insufficient, and when the cumulative reduction ratio is greater than 90%, the rolling deformation resistance is greatly increased, thereby increasing the manufacturing cost.
  • the bar plate is sequentially removed through a first row of spraying the coolant at a pressure of 50 to 150 bar and two rows of spraying the coolant at a pressure of 100 to 250 bar to remove scale.
  • the surface scale thickness can be removed up to 50 ⁇ m using a single row and two row nozzles of a Finishing Mill Scale Breaker (FSB) prior to finishing rolling the bar plate.
  • FFB Finishing Mill Scale Breaker
  • the pressure of the first row and the second row nozzles is less than 50 bar and 100 bar, respectively, the scale is insufficient to be removed, and a large amount of fusiform and scale scales are generated on the surface of the steel sheet after finishing rolling, resulting in inferior surface quality after pickling.
  • the pressure of the first row nozzle is more than 150bar or the pressure of the second row nozzle is more than 250bar, the finish rolling temperature is too low to obtain an effective austenite fraction, it is difficult to secure the target tensile strength.
  • the bar plate from which the scale is removed is finish rolled at a temperature range of Ar1 to Ar3 to obtain a hot rolled steel sheet.
  • it can finish-roll in the finishing mill which consists of 3-6 stands.
  • a high fraction of precipitates is formed in the finish rolling process, thereby reducing the grain size by reducing the grain strengthening effect by reducing the fraction of precipitates to be precipitated finely at a low temperature in the temperature range of Ar1 to Ar3. To compensate with an increase in strengthening effect.
  • the finish rolling may be performed so that the sheet speed is 200 ⁇ 600mpm, the thickness of the hot rolled steel sheet is 3.0mm or less.
  • finishing rolling speed is more than 600mpm, an operation accident such as a panda stage may occur, and isothermal and constant isothermal rolling may be difficult, so that a uniform temperature may not be secured and material deviation may occur.
  • isothermal and constant isothermal rolling may be difficult, so that a uniform temperature may not be secured and material deviation may occur.
  • less than 200mpm it may be difficult to secure the finish rolling temperature because the finish rolling speed is too slow.
  • finish rolling may be performed such that 20-40% of ferrite and 60-80% of austenite are formed in an area fraction.
  • the finish rolling can be performed so that the rolling speed difference in one bar plate is 10% or less.
  • High-strength steel of 780MPa class for the purpose of the present invention is very likely to change the material properties according to the deformation rate during the finish rolling because the formation of the transformation structure as a reinforcing mechanism.
  • the difference in rolling speed exceeds 10% in a finishing mill consisting of a plurality of stands, it is difficult to obtain a uniform cooling rate and a target winding temperature in a subsequent run out table, and thus the width or length of the strip. This may cause a large material deviation in the direction.
  • the hot rolled steel sheet After cooling the hot rolled steel sheet for 3 to 8 seconds, it is cooled at a cooling rate of 200 ° C / s or more and wound up to 250 ° C or less.
  • the concentration of C to the residual austenite is insufficient and the time for ferrite transformation is insufficient, which increases the risk of deterioration of elongation. Not only is there difficulty in securing strength, but also the length of the installation may be long or productivity may be reduced.
  • the cooling rate is slower than 200 ° C / sec is promoted ferrite transformation and cementite is difficult to obtain the desired material.
  • the coiling temperature is 250 ° C or higher, it is difficult to obtain martensite structure, and martensite obtained by cooling may be auto tempered, thereby making it difficult to obtain a target tensile strength.
  • the pickled hot rolled steel sheet may further comprise the step of obtaining a PO material.
  • the pickling treatment which can be used in the present invention is not particularly limited since it can be applied as long as it is a treatment method generally used in the hot acid pickling process.
  • the molten steel having the component composition shown in Table 1 below was prepared.
  • Ar1 and Ar3 temperature in Table 2 is the value computed using the commercial thermodynamic software JmatPro V-8.
  • the area fractions of ferrite (F), martensite (M) and bainite (B) were measured using a scanning electron microscope (SEM).
  • Ferrite Grain Size was measured at random diameters of 10 spots at 3,000x magnification by using EBSD (Electron BackScatter Diffraction), and then measured by the equivalent diameter using Image-Plus Pro software. One average value is listed.
  • Tensile strength is the value obtained by taking JIS 5 specimens at the width W / 4 point in the rolling direction. Material deviation is the maximum value minus the minimum value of the tensile strength measured in the width and length direction of the coil. will be.
  • Evaluation criteria for the surface quality of the PO material is as follows.
  • RSB Rolling Mill Scale Breaker, rough rolling scale brake
  • FSB Finishing Mill Scale Breaker, finish rolling scale brake
  • Inventive examples 1 and 2 satisfying all the conditions presented in the present invention satisfy the target tensile strength (780 MPa or more) and elongation (15% or more), and it can be seen that both the edge surface quality and the surface quality of the PO material are excellent. have.
  • Example 1 is a photograph taken with a scanning electron microscope (SEM) of the microstructure of Inventive Example 2. It is confirmed that ferrite (F) and martensite (M) are composed of a main phase, and some bainite (B) is present.
  • SEM scanning electron microscope
  • FIG. 2 is a graph showing the ferrite grain size distribution of Inventive Example 2 measured using EBSD. FIG. From this result, it can be confirmed that the crystal grains of 1-3 micrometers or less are minutely distributed, and the average grain size is 1.95 micrometers.
  • Figure 3 shows the surface photograph of the PO material obtained by pickling the hot rolled steel sheet of Inventive Example 2, it can be confirmed that the surface quality is excellent.
  • Comparative Examples 1 to 5 were satisfied with the alloy composition proposed in the present invention, but did not satisfy the manufacturing conditions did not secure the target material.
  • Comparative Examples 1 and 2 did not satisfy the cooling conditions, the strength was inferior.
  • Comparative Example 3 the edge quality was inferior when the rough rolling side edge temperature was less than that.
  • Comparative Examples 4 and 5 did not satisfy the RSB or FSB pressure presented in the present invention, the surface quality was inferior.
  • Comparative Example 6 all of the operating conditions were the same as in Inventive Example 2, but the Si content was exceeded, and thus, the target tensile strength was not satisfied, and the PO material surface quality was also inferior.
  • Comparative Example 7 did not satisfy the target tensile strength when the Cr content was low.
  • Comparative Example 8 the elongation was inferior when the Si content was low.

Abstract

L'objectif de la présente invention est d'obtenir une plaque d'acier laminée à chaud de résistance élevée et son procédé de fabrication, où, pour surmonter le problème des procédés de mini-broyage existants, un mode de roulement sans fin est utilisé dans un procédé qui combine directement la coulée continue et le laminage, ce qui permet d'obtenir une plaque d'acier laminée à chaud qui présente une excellente qualité de surface, des écarts de matière sensiblement réduits dans les directions transversale et longitudinale de celle-ci, et une résistance à la traction d'environ 780 MPa.
PCT/KR2017/014963 2016-12-23 2017-12-18 Plaque d'acier laminée à chaud de résistance élevée ayant un faible écart de matière et une excellente qualité de surface et son procédé de fabrication WO2018117575A1 (fr)

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KR10-2016-0177937 2016-12-23

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CN110100033B (zh) 2021-04-20

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