WO2019009677A1 - Tôle d'acier laminée à chaud de haute résistance perfectionnée et son procédé de fabrication - Google Patents

Tôle d'acier laminée à chaud de haute résistance perfectionnée et son procédé de fabrication Download PDF

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Publication number
WO2019009677A1
WO2019009677A1 PCT/KR2018/007721 KR2018007721W WO2019009677A1 WO 2019009677 A1 WO2019009677 A1 WO 2019009677A1 KR 2018007721 W KR2018007721 W KR 2018007721W WO 2019009677 A1 WO2019009677 A1 WO 2019009677A1
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steel sheet
rolled steel
hot
high strength
strength hot
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PCT/KR2018/007721
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English (en)
Korean (ko)
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공종판
정제숙
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주식회사 포스코
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Priority to CN201880044939.4A priority Critical patent/CN110869526B/zh
Publication of WO2019009677A1 publication Critical patent/WO2019009677A1/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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/04—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for de-scaling, e.g. by brushing
    • B21B45/08—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for de-scaling, e.g. by brushing hydraulically
    • 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
    • C21D8/0247—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
    • 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
    • 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
    • C22C38/28—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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 super high strength hot-rolled steel sheet and a method of manufacturing the same, and more particularly, to a super high-strength hot-rolled steel sheet having a high tensile strength of 980 MPa, To a hot-rolled steel sheet and a manufacturing method thereof.
  • AHSS AHSS
  • a steel sheet to be applied to an automobile body is required not only to have high strength and high ductility, but also to have good weldability and welded part mechanical properties.
  • the so-called DP (dual phase) steel is a steel mainly composed of two phases of ferrite and martensite, and is one of the representative steels having a low yield strength.
  • Patent Documents 1 and 2 The technology related to the production of such AHSS hot-rolled steel is disclosed in Patent Documents 1 and 2.
  • all of these methods relate to a method of manufacturing in a conventional hot-melt mill, It is difficult to avoid.
  • the target material is to be produced at a low temperature below Ms (martensitic transformation starting temperature)
  • Ms martensitic transformation starting temperature
  • Patent Document 3 relates to a method for producing a hot-rolled DP steel having a tensile strength of 590 MPa by batch method in a mini-mill process, and the final steel sheet thickness is about 3.0 mm.
  • Patent Document 1 U.S. Patent No. 4285741
  • Patent Document 2 U.S. Patent No. 4325751
  • Patent Document 3 Korean Published Patent Application No. 10-2012-0052022
  • a preferred aspect of the present invention is to provide an ultrahigh-strength hot-rolled steel sheet having excellent surface quality and weldability and being significantly reduced in width, lengthwise material deviation, and edge cracks.
  • Another aspect of the present invention is to provide a method of manufacturing an ultra-high-strength hot-rolled steel sheet excellent in surface quality and weldability, and significantly reduced in width, lengthwise material deviation, and edge cracks, by a performance- .
  • a method of manufacturing a steel sheet which is produced by a performance-rolling direct process, comprising 0.030 to 0.085% of C, 1.8 to 3.0% of Mn, 0.03 to 1.0% of Si, 0.005 to 0.05% , S: 0.01% or less, Cr: 0.2 to 2.0%, Al: 0.01 to 0.07%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.0050%, N: 0.001 to 0.010%, and other Fe and other unavoidable impurities And an ultrahigh-strength hot-rolled steel sheet having a microstructure containing 40 to 60% of ferrite, 30 to 50% of martensite and 10 to 20% of bainite in an areal fraction.
  • Al and Ti can satisfy the following relational expression (1).
  • Ceq expressed by the following formula (2) may be 0.18 to 0.28.
  • the hot-rolled steel sheet may contain at least one of Cu, Ni, Mo, Sn, and Pb as a tramp element, and the total content thereof may be 0.2 wt% or less.
  • the bainite may be formed at the interface between ferrite and martensite.
  • the mean size of the ferrite crystal grains measured by the circle equivalent diameter may be 5 ⁇ or less.
  • the hot-rolled steel sheet may comprise 25-1000 / ⁇ m the Ti (C, N) precipitates, the Ti (C, N) average size of the precipitate can be 50nm or less in circle equivalent diameter.
  • the rolling process includes a rough rolling process for producing a bar plate, and a molar fraction (%) of an AlN precipitate at an edge portion of the roughly rolled bar plate may satisfy the following formula (3) .
  • the hot-rolled steel sheet may have a tensile strength of 980 MPa or more, an elongation of 10% or more, and a deviation (material deviation) of tensile strength of 30 MPa or less.
  • the thickness of the hot-rolled steel sheet may be 3.0 mm or less.
  • a method of manufacturing a steel sheet which is manufactured through a performance-rolling direct process, and which comprises 0.030 to 0.085% of C, 1.8 to 3.0% of C, 0.03 to 1.0% of Si, 0.005 0.001 to 0.050% of N, 0.001 to 0.010% of N, 0.001 to 0.010% of Cr, 0.2 to 2.0% of Cr, 0.01 to 0.07% of Al, 0.005 to 0.10% of Ti, And has a microstructure containing an impurity and an area fraction of 40 to 60% of ferrite, 30 to 50% of martensite, and 10 to 20% of bainite, and in the case of resistance spot welding after pickling treatment,
  • Al and Ti can satisfy the following relational expression (1).
  • Ceq expressed by the following formula (2) may be 0.18 to 0.28.
  • the hot-rolled steel sheet may contain at least one of Cu, Ni, Mo, Sn, and Pb as a tramp element, and the total content thereof may be 0.2 wt% or less.
  • the Vickers hardness of the fused portion corresponding to the nugget may be 350 to 450 Hv, and the difference between the hardness of the base material and the minimum hardness of the softened portion may be 100 Hv or less.
  • the microstructure of the molten portion corresponding to the nugget may include a martensite structure having an area ratio of 95% or more.
  • the rolling process includes a rough rolling process for producing a bar plate, and the molar fraction (%) of the AlN precipitate at the edge portion of the roughly rolled bar plate can satisfy the following formula (3).
  • a method of manufacturing a hot-rolled steel sheet by a direct-rolling-performance-rolling process comprising the steps of: C: 0.030-0.085%; Mn: 1.8-3.0%; Si: 0.001 to 0.05% of P, 0.2 to 2.0% of Cr, 0.01 to 0.07% of Al, 0.005 to 0.10% of Ti, 0.0005 to 0.0050% of B, 0.001 to 0.010% of N, Continuously cast molten steel containing remaining Fe and other unavoidable impurities into a slab having a thickness of 60 to 120 mm;
  • Al and Ti can satisfy the following relational expression (1).
  • Ceq expressed by the following formula (2) may be 0.18 to 0.28.
  • the molten steel may contain at least one of Cu, Ni, Mo, Sn and Pb as a tramp element, and the total content thereof may be 0.2 wt% or less.
  • the casting speed of the continuous casting may be 4 to 8 mpm.
  • the slab may be heated before the surface descaling step, and the slab heating temperature may be 900 to 1200 ° C.
  • the surface temperature of the rough-rolled slab at the time of rough rolling may be 900 to 1200 ° C, and the edge temperature of the rough-rolled bar-plate edge may be 780 to 1100 ° C.
  • the temperature of the rough rolling out side bar plate edge portion can be controlled so that the AlN mole fraction (%) precipitated at the bar plate edge portion temperature satisfies the following formula (3).
  • the cumulative rolling reduction during the rough rolling may be 60 to 90%.
  • the sheet passing speed may be 200 to 600 mpm, and the thickness of the hot-rolled steel sheet produced by the finish rolling may be 3.0 mm or less.
  • the rolling speed difference between the top and the tail of the steel sheet during the finish rolling may be 10% or less.
  • the present invention not only the surface quality is improved by using the continuous continuous rolling mode in the performance-rolling direct process, but also the material variation in the width and length direction of the steel sheet is remarkably reduced, and the yield rate is excellent. , It is possible to produce an ultra-high strength hot-rolled steel sheet having a thickness of 3.0 mm or less and a tensile strength of 980 MPa. In addition, edge cracks can be significantly reduced.
  • the hot-rolled steel sheet produced by the present invention is superior in the quality of the edges and the surface scale, and is capable of producing advanced PO by a general hot-rolling pickling process, It can be differentiated from existing minilmill and hot-milling processes that can only be produced by itself, so it is excellent in terms of price competition and can significantly improve value-added.
  • Fig. 3 shows the ferrite grain size distribution of Inventive Example 7.
  • Example 4 is a transmission electron microscope (TEM) micrograph of the precipitate of Inventive Example 7 taken.
  • Fig. 6 shows the distribution of the precipitate size (nm) in Inventive Example 7.
  • Fig. 9 shows a state diagram of the inventive example 7.
  • FIG. 11 is a graph showing changes in mole fraction of AlN / TiN according to the temperature of Inventive Example 7.
  • Example 13 is a graph showing the change in the mole fraction of AlN according to the temperature of Conventional Example 1.
  • 17 is a SEM micrograph of the molten portion of the present invention.
  • Figure 18 shows a lay-out of a preferred example of a performance-rolling direct process that the present invention may be applied to.
  • Figure 19 shows another preferred lay-out of a performance-rolling direct process in which the present invention may be applied.
  • the present inventors conducted research and experiments on an ultra-high-strength hot-rolled steel sheet excellent in surface quality and weldability, and significantly reduced in width, lengthwise material deviation, and edge cracks through a performance-rolling direct process, The present invention has been completed.
  • the present invention relates to an ultrahigh-strength hot-rolled steel sheet excellent in surface quality and weldability by properly controlling steel composition, microstructure and manufacturing conditions in a performance-rolling direct-cutting process, and significantly reducing width, lengthwise material deviation and edge cracks, And a method for manufacturing the same.
  • a super high strength hot-rolled steel sheet is manufactured by a performance-rolling direct process, and comprises 0.030 to 0.085% of C, 1.8 to 3.0% of C, 0.03 to 1.0% of Si, 0.005 to 0.05%, S: 0.01% or less, Cr: 0.2 to 2.0%, Al: 0.01 to 0.07%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.0050% Contains inevitable impurities, and has a microstructure including an area fraction of 40 to 60% of ferrite, 30 to 50% of martensite and 10 to 20% of bainite.
  • the alloy composition of the present invention will be described in detail.
  • the unit of each element content is% by weight.
  • Carbon (C) is a very important element for increasing the strength of a steel sheet and securing a composite structure composed of ferrite and martensite.
  • the C content is less than 0.030%, it may be difficult to secure the desired strength in the present invention.
  • an apodization reaction L + Delta Ferrite ⁇ Austentite
  • the C content is preferably 0.03 to 0.085%.
  • Manganese (Mn) is an element that has a very strong effect of solid solution strengthening, and at the same time promotes the formation of composite structure composed of ferrite and martensite. If the Mn content is less than 1.8%, it may be difficult to obtain the desired strength in the present invention. On the other hand, when the Mn content exceeds 3.0%, alloy steel cost rise and weldability and hot rolling property may be weakened. Therefore, the Mn content is preferably 1.8 to 3.0%. , More preferably 1.9 to 2.8%, and even more preferably 2.0 to 2.6%.
  • Silicon (Si) is a useful element that can secure strength without deteriorating the ductility of the steel sheet. It is also an element promoting the formation of martensite by promoting ferrite formation and promoting C concentration in untransformed austenite. When the Si content is less than 0.03%, it is difficult to sufficiently secure the above effect. On the other hand, when the Si content is more than 1.0%, the scale of the steel is generated on the surface of the steel sheet, and traces remain on the surface of the steel sheet after pickling, and the surface quality may be deteriorated. Therefore, the Si content is preferably 0.03 to 1.0%. , More preferably 0.030 to 0.80%, and even more preferably 0.035 to 0.50%.
  • Phosphorus (P) is an element having an effect of strengthening the steel sheet.
  • the P content is less than 0.005%, it is difficult to secure the effect.
  • the P content exceeds 0.05%, the grain boundary and / or the intergranular grain boundary may be segregated to cause brittleness. Therefore, the content of P is preferably limited to 0.005 to 0.05%. , More preferably 0.006 to 0.040%, and even more preferably 0.010 to 0.025%.
  • S Sulfur
  • S is an impurity which segregates during MnS nonmetallic inclusions and performance solidification in steel and can cause high temperature cracks. Therefore, the content thereof should be controlled as low as possible and preferably controlled to be 0.01% or less.
  • Chromium (Cr) is an element that improves hardenability and increases the strength of steel.
  • the Cr content is preferably 0.2 to 2.0%. , More preferably 0.3 to 1.8%, and even more preferably 0.5 to 1.4%.
  • Aluminum (Al) plays a role in suppressing the formation of carbides and increasing the ductility of steel.
  • the Al content is less than 0.01%, the above-mentioned effect is insufficient.
  • the Al content is more than 0.07%, a large amount of AlN precipitates are formed to deteriorate the edge quality of the cast steel or bar plate due to deterioration of high temperature ductility, and the steel can be thickened on the surface of the steel plate to deteriorate the plating ability. Therefore, the Al content is preferably 0.01 to 0.07%. , More preferably 0.015 to 0.06%, and even more preferably 0.02 to 0.05%.
  • Titanium (Ti) is an element for forming precipitates and nitrides, which increases the strength of steel.
  • Ti is an element that reduces the sensitivity of edge cracking by preventing the deterioration of high-temperature ductility by reducing the amount of AlN precipitate by removing solute N through the formation of TiN near the solidification temperature.
  • the Ti content is preferably 0.005 to 0.10%. , More preferably 0.008 to 0.08%, and even more preferably 0.01 to 0.075%.
  • B Boron
  • the B content is preferably 0.0005 to 0.0050%. , More preferably 0.001 to 0.0040%, and even more preferably 0.001 to 0.0025%.
  • Nitrogen (N) is an austenite stabilizing and nitriding element.
  • the N content is preferably 0.001 to 0.010%. , More preferably 0.0025 to 0.0095%, and even more preferably 0.0040 to 0.0090%.
  • Al and Ti can satisfy the following relational expression (1).
  • the above Ti, Al and N not only satisfy the above-mentioned numerical ranges but can be controlled so as to satisfy the above formula (1) in order to improve the surface quality and significantly reduce the edge crack while securing high strength.
  • Aluminum (Al) in the steel reacts with nitrogen (N) to form AlN precipitates.
  • the slab / bar plate cracks are induced in the slab cooling conditions under which these precipitates are precipitated during the production of thin slabs to lower the edge quality of the slab or hot- It is possible to appropriately add Ti to remove solute N through the formation of TiN near the solidification temperature of molten steel to reduce the amount of AlN precipitate to control the defect.
  • the steel sheet of the present invention has excellent RSW weldability and weld material properties.
  • the steel sheet of the present invention not only satisfies the alloy composition described above, but also has Ceq of 0.18 to 0.28 expressed by the following formula (2).
  • the above formula (2) is a component relational expression for securing the weldability of the steel sheet.
  • the Ceq value is more preferably from 0.18 to 0.27, and still more preferably from 0.18 to 0.26.
  • Ceq is less than 0.18, the curing ability is low and it may be difficult to secure the target tensile strength.
  • Ceq exceeds 0.28 the weldability is lowered and the physical properties of the welded portion may deteriorate. More preferably 0.18 to 0.27, and even more preferably 0.19 to 0.26.
  • the remainder of the present invention is iron (Fe).
  • impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.
  • the tram element is an impurity element derived from scrap used as a raw material in a steelmaking process. If the total amount exceeds 0.2%, surface cracking of the thin slab and surface quality of the hot-rolled steel sheet may be deteriorated.
  • the fraction of ferrite and martensite combined is 80% or more in area fraction, and the remainder is composed of bainite structure.
  • it contains 40 to 60% of ferrite in an area fraction, 30 to 50% of martensite and 10 to 20% of bainite.
  • the ferrite fraction exceeds 60%, it is difficult to secure the desired strength.
  • the ferrite fraction is less than 40%, the fraction of the remaining martensite and bainite structure increases, which is difficult to secure the ductility.
  • the martensite fraction exceeds 50%, the strength becomes too high to secure the ductility.
  • the martensite fraction is less than 30%, it may be difficult to secure the desired strength.
  • the DP (dual phase) composed of two phases of ferrite + martensite has a high martensite fraction, so that martensite is tempered in the heat affected zone during welding, resulting in softening phenomenon and a decrease in strength.
  • the bainite structure is secured to some extent instead of the martensite, such problems can be solved and the strength and workability can be secured simultaneously due to the bainite structure characteristics.
  • bainite is a structure having an intermediate strength between ferrite and martensite, and when the bainite structure is formed at these two texture interfaces, the above problems can be improved and the workability can be improved.
  • the average size of the ferrite grains measured by the circle equivalent diameter may be 5 ⁇ or less. More preferably not more than 4 mu m, and even more preferably not more than 3 mu m. In order to simultaneously secure strength and workability through securing a ferrite structure having fine crystal grains, when the size of the ferrite crystal grains exceeds 5 ⁇ , it may be difficult to secure the desired strength and workability.
  • the hot-rolled steel sheet of the present invention may contain 5 to 1000 Ti / 2 ⁇ 2 precipitates, more preferably 5 to 500 Ti / 2 ⁇ 2 , and still more preferably 5 To 200 pieces / ⁇ 2 , and the Ti (C, N) precipitate may have an average size of 50 nm or less in circle equivalent diameter.
  • Ti (C, N) precipitates are meant to include TiN, Ti (C, N), TiC and complex precipitates thereof.
  • the size of the precipitate exceeds 50 nm, it may be difficult to effectively secure the strength.
  • the number of precipitates is less than 5 / ⁇ 2 , it may be difficult to secure a desired strength.
  • the number of precipitates is more than 1000 / ⁇ 2 , the elongation rate is lowered due to the increase of the strength, and processing may become difficult.
  • the molar fraction (%) of the AlN precipitate at the edge portion of the rough- (3) can be satisfied.
  • the thickness of the hot-rolled steel sheet of the present invention may be 3.0 mm or less, preferably 2.0 mm or less, and more preferably 1.5 mm or less.
  • the hot-rolled steel sheet of the present invention may have a tensile strength of 980 MPa or more, an elongation of 10% or more, a deviation (material variation) of tensile strength of 30 MPa or less, and preferably 20 MPa or less.
  • a super high strength hot-rolled steel sheet is manufactured by a performance-rolling direct process, and includes 0.030 to 0.085% of C, 1.8 to 3.0% of C, 0.03 to 1.0% of Si, 0.03 to 1.0% of C, : 0.005 to 0.05%, S: 0.01% or less, Cr: 0.2 to 2.0%, Al: 0.01 to 0.07%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.0050% And other inevitable impurities and has a microstructure containing 40 to 60% of ferrite in an area fraction, 30 to 50% of martensite and 10 to 20% of bainite, and in resistance point welding after pickling treatment,
  • the appropriate welding range in resistance spot welding is a key index for evaluating the weldability and can be defined as the range between the upper limit current and the lower limit current.
  • the optimum welding range between the lower current satisfying the minimum nugget diameter of 4t 1/2 (t is the base material thickness (mm)) and the upper current causing the expansion is more than 1.5 (kA), and the minimum nugget diameter is 5t 1 / 2
  • the appropriate welding range between the lower current limit and the upper current limit may be 1.0 (KA) or more.
  • the minimum nugget diameter 4t 1/2 means a minimum nugget diameter required to switch to the pull-out fracture (Pull Out Fracture, POF) at the interface fracture (Fracture Interfacial, IF).
  • the upper limit current is defined as the value obtained by subtracting 0.2 (kA) from the current generated by scattering.
  • the ductility ratio (CTS / TSS) is defined as the ratio of Tensile Shear Strength (TSS) to Cross Tensile Strength (CTS), and is a comprehensive index that determines the mechanical properties of the weld in the AHSS RSW .
  • the ductility ratio of the developed steel may be at least 35% over a given appropriate welding current range. If it is less than 35%, the passenger may not be able to stably protect the passenger in the event of a body collision in order to open the weld strength characteristic and the collision stability.
  • the Vickers hardness of the fusion zone which is the RSW nugget of the steel sheet of the present invention, is 350 to 450 Hv. If it is lower than 350Hv, sufficient hardness of the molten part can not be secured and the strength of the welded part may be low. However, when it exceeds 450 Hv, the strength of the welded portion and especially the impact absorption energy may be low because the hardness of the molten portion is too high, the sensitivity of cracking is high, and the interfacial fracture where fracture occurs at the nugget interface becomes high.
  • the Vickers hardness of the fusion zone is more preferably 360 to 440 Hv, and even more preferably 370 to 430 Hv.
  • the softened part is a typical phenomenon observed in the AHSS welded part having a high martensite fraction, and when the softened part hardness is too low, the softened part may be judged and the strength of the welded part may be low. Accordingly, in the present invention, the difference between the hardness of the base metal and the minimum hardness of the softening zone may be 100 Hv or less. More preferably 90 Hv or less, and even more preferably 80 Hv or less. If it is more than this value, breakage occurs in the softened portion, and it may be difficult to secure sufficient strength of the welded portion.
  • the microstructure of the nugget (melting portion) of the RSW of the inventive steel sheet preferably has a martensite structure in the nugget of 95% or more.
  • a method of manufacturing a hot-rolled steel sheet by a direct-rolling process which comprises continuously casting molten steel having the alloy composition described above into a slab having a thickness of 60 to 120 mm ;
  • the molten steel having the above-mentioned alloy composition is continuously cast to prepare a slab having a thickness of 60 to 120 mm.
  • the casting speed is preferably set to, for example, 4 to 8 mpm.
  • the reason why the casting speed is preferably 4 mpm or more is because a high speed casting and a rolling process are connected to each other and a casting speed higher than a certain level is required to secure the target rolling temperature.
  • the casting speed is slow, there is a risk of segregation from the cast steel. If such segregation occurs, it is difficult to secure strength and workability, and the risk of material variation in the width direction or the longitudinal direction is increased.
  • the operation success rate may be reduced due to instability of the molten steel bath surface, so that the casting speed is preferably set to 4 to 8 mpm. More preferably 4.2 to 7.2 mpm, and even more preferably 4.5 to 6.5 mpm.
  • the thickness of the slab is more than 120 mm, high-speed casting is difficult, and the rolling load during rough rolling is increased.
  • the slab thickness is less than 60 mm, the temperature of the cast steel is rapidly decreased.
  • the thickness of the slab is limited to 60 to 120 mm. More preferably 70 to 110 mm, and even more preferably 80 to 100 mm.
  • the slab may be heated before the surface descaling step, and the slab heating temperature may be 900 to 1200 ° C.
  • the surface temperature of the slab is less than 900 ⁇ , there is a possibility that cracks are generated in the edge portion of the bar plate in the course of an increase in the rough rolling load and in the rough rolling. In this case, the edge of the hot rolled steel sheet may be defective. If the slab surface temperature exceeds 1200 ° C, problems such as deterioration of hot rolling surface quality due to the remnant of a hot rolling scale may occur. Therefore, the heating temperature of the slab is preferably in the range of 900 to 1200 ° C. More preferably 950 to 1150 ⁇ ⁇ , and still more preferably 1000 to 1100 ⁇ ⁇ .
  • the scale is removed by injecting cooling water at a pressure of 50 to 350 bar into the cast slab or the heated slab after casting.
  • cooling water of 50 ° C or less is sprayed at a pressure of 50 to 350 bar in a roughing scale breaker (hereinafter, referred to as 'RSB') to obtain a surface scale thickness of, for example, 300 ⁇ m or less
  • 'RSB' roughing scale breaker
  • the scale can be removed. If the pressure is less than 50 bar, a large amount of arithmetic scale scale or the like may remain on the surface of the slab and the surface quality may become dull after pickling. On the other hand, if the temperature exceeds 350 bar, the edge temperature of the bar plate may drop rapidly, and an edge crack may occur.
  • the cooling water injection pressure may be more preferably 100 to 300 bar, and still more preferably 150 to 250 bar.
  • the scale-removed slab is rough-rolled, for example, in a roughing mill composed of 2 to 5 stands to obtain a bar plate.
  • the surface temperature of the slab at the inlet side of the roughing mill is preferably in the range of 900 to 1200 ° C.
  • the surface temperature of the slab is less than 900 ⁇ , there is a possibility that cracks are generated in the edge portion of the bar plate in the course of an increase in the rough rolling load and in the rough rolling. In this case, the edge of the hot rolled steel sheet may be defective.
  • the slab surface temperature exceeds 1200 ° C, problems such as deterioration of hot rolling surface quality due to the remnant of a hot rolling scale may occur. Therefore, the surface temperature of the slab preferably ranges from 900 to 1200 ° C. More preferably 950 to 1150 ⁇ ⁇ , and still more preferably 1000 to 1100 ⁇ ⁇ .
  • the edge temperature of the rough-rolled bar plate at the time of rough rolling has a range of 780 to 1100 ° C. If the edge portion temperature is less than 780 ° C, the AlN precipitates precipitate and the susceptibility to edge cracking may become very high as the high temperature ductility deteriorates. On the other hand, when the edge temperature exceeds 1100 ° C, the temperature of the center of the thin slab becomes too high, so that a large number of arithmetic scale may occur and the surface quality may become poor after pickling.
  • the temperature at the edge of the bar plate at the time of rough rolling during the rough rolling may be more preferably 800 to 1080 ° C, and still more preferably 820 to 1060 ° C.
  • the edge temperature of the bar-rolling-out side bar plate not only satisfies the above-described respective numerical ranges but also satisfies the following formula (3) in order to improve surface and edge quality while ensuring high strength.
  • cumulative rolling reduction during rough rolling plays an important role in obtaining a uniform target material in the present invention. That is, as the reduction rate in rough rolling increases, microstructures such as Mn, Si, and Cr, which are important elements in the manufacture of high strength steel, become uniform and the temperature gradient in the width and thickness direction of the strip becomes smaller. Valid. However, if the cumulative rolling reduction is less than 60%, the above effects can not be sufficiently exhibited. If the cumulative rolling reduction exceeds 90%, the rolling resistance increases greatly and the manufacturing cost rises. It is preferable to perform rolling. , More preferably 65 to 85%, and still more preferably 70 to 80%.
  • the scale is removed by injecting cooling water into the bar plate at a pressure of 50 to 350 bar.
  • cooling water of 50 ° C or less is sprayed at a pressure of 50 to 350 bar in a Finishing Mill Scale Breaker (FSB) nozzle, so that the surface scale thickness is, for example,
  • FFB Finishing Mill Scale Breaker
  • the scale can be removed so as to be not more than 30 mu m.
  • the cooling water injection pressure is more preferably 100 to 300 bar, and even more preferably 150 to 250 bar.
  • the bar plate on which the scale has been removed is finish-rolled in a finishing mill composed of a plurality of stands, for example, three to six stands.
  • the ultrahigh strength steel of 980 MPa class which is the object of the present invention, utilizes the formation of the transformed structure as an strengthening mechanism, there is a high possibility that the material properties change according to the strain rate during finish rolling. That is, when the rolling speed difference between the top and the tail of the steel sheet exceeds 10% during finishing rolling in a finishing mill composed of a plurality of stands, a uniform cooling rate and a uniform cooling rate in a subsequent run- It is difficult to obtain the target coiling temperature, which results in causing a material variation in the width or length direction of the strip. Therefore, it is preferable to control so that the difference in rolling speed between the top of the steel sheet and the tail in the finishing rolling step does not exceed 10%.
  • the finishing rolling temperature is preferably Ar 3 -30 ⁇ to Ar 3 + 60 ⁇ .
  • the finish rolling temperature is lower than Ar 3 -30 ° C, the load of the roll during hot rolling is greatly increased to increase the energy consumption and the working speed, and since a sufficient austenite fraction can not be secured, the target microstructure and material are secured Can not.
  • the finish rolling temperature is higher than Ar 3 + 60 ° C, the crystal grains can not cooperate with each other to obtain a high strength. In order to obtain a sufficient bainite and martensite structure, the cooling rate must be further increased.
  • the finishing rolling temperature is more preferably Ar 3 -20 ⁇ to Ar 3 + 50 ⁇ .
  • the finish rolling can be performed such that the sheet passing speed is 200 to 600 mpm and the thickness of the hot-rolled steel sheet is 3.0 mm or less. Preferably 2.0 mm or less, and more preferably 1.6 mm or less.
  • finish rolling speed exceeds 600 mPm, it is possible to cause an operation accident such as plate breakage, and because isothermal constant velocity rolling is difficult, uniform temperature can not be ensured and material deviation may occur. On the other hand, in the case of less than 200 mpm, the finish rolling speed is too slow to secure the finishing rolling temperature.
  • the finish-rolled hot-rolled steel sheet is, for example, air-cooled for 1 to 8 seconds in a run-out table, cooled to a martensitic transformation completion temperature (Mf) or less at a cooling rate of 150 ° C / sec or more and wound.
  • Mf martensitic transformation completion temperature
  • the cooled hot rolled steel sheet undergoes air cooling for 1 to 8 seconds on the runout table. If the time is less than 1 second, the C concentration in the retained austenite is insufficient and the time for ferrite transformation is insufficient, And if it exceeds 8 seconds, there is a problem in securing a desired tensile strength due to excessive transformation of ferrite, and also a problem that a length of the equipment is long or productivity is lowered. Therefore, the air- To 8 seconds.
  • the air cooling time is more preferably 1.5 to 6.5 seconds, and still more preferably 2.0 to 5.0 seconds.
  • the cooling rate during the cooling after the air cooling is 150 ° C / sec or more, and the coiling temperature is preferably not higher than the martensitic transformation completion temperature (Mf).
  • the cooling rate is slower than 150 ° C / sec, ferrite transformation is promoted and cementite is formed, making it difficult to obtain a desired material. Further, when the coiling temperature exceeds the martensitic transformation completion temperature (Mf), it is difficult to obtain a sufficient martensite structure and the martensite obtained by cooling can be auto tempered to obtain a desired tensile strength Can be difficult.
  • Mf martensitic transformation completion temperature
  • the pickling treatment that can be used in the present invention is not particularly limited as long as it is generally applicable to a treatment method used in a hot-rolling pickling process.
  • the high-strength hot-rolled steel sheet having a small material deviation of 980 MPa and excellent in surface quality according to one aspect of the present invention can be manufactured through the performance-rolling direct-joining process as shown in FIGS. 18 and 19.
  • the performance-to-rolling direct process layout comprises a high-speed continuous casting machine 100 for producing slabs (a) of a first thickness and a high-speed continuous casting machine 100 for producing slabs of a second thickness
  • a thin slab a having a thickness of 60 to 120 mm is manufactured in the continuous casting machine 100 and the bar plate b is further heated in the heater 200 to ensure a sufficient finish rolling temperature.
  • a roughing scale breaker 300 hereinafter, referred to as 'RSB'
  • a finishing mill scale breaker (FSB) 500 are placed in front of the finishing mill 600 in front of the roughing mill 400 (Pickled & Oiled), which is excellent in surface quality when picking hot-rolled steel sheets in a later process, is also possible.
  • the layout of the performance-rolling direct process includes the slab addition heater 200 in front of the roughing mill 400 in addition to the one shown in FIG. 18, whereby the slab edge temperature can be easily secured and the occurrence of edge defects can be reduced It is advantageous for ensuring surface quality.
  • a space of at least one slab length is secured before the roughing mill, and batch rolling is also possible.
  • Table 1 Molten steel having the composition shown in Table 1 below was prepared.
  • Table 1 below shows the C content of the initiation of the apodization reaction and is a value calculated using Thermo-Calc-3.0.1 Console Mode (Database: TCFE6), a commercial thermodynamic software.
  • a hot-rolled steel sheet having a thickness of 1.6 mm was produced in the continuous rolling mode in the performance-to-rolling direct connection process according to the production conditions shown in Table 2 below (Inventive Example 1 -7 and Comparative Examples 1 to 11).
  • a hot-rolled steel sheet having a thickness of 3.2 mm was produced in the batch mode in the conventional mini-mill process by applying the manufacturing conditions shown in Table 2.
  • RSB Rolling Mill Scale Breaker, rough rolling scale brake
  • FSB Feishing Mill Scale Breaker, finishing rolling scale brake
  • the microstructure phase fraction, ferrite grain size (FGS), yield strength (YS), tensile strength Table 3 shows the results of measuring elongation (EL), deviation (material deviation) (? TS) of tensile strength, occurrence of edge cracks and PO surface quality.
  • the area fraction of ferrite (F), martensite (M) and bainite (B) was determined by scanning electron microscope (SEM) (FGS, Ferrite Grain Size) of 10 areas randomly photographed at a magnification of 3,000 times using Scanning Electron Microscope, image area ratio measurement using Image-Plus Pro software, Shows an average value measured at a circle-equivalent diameter using Image-Plus Pro software after randomly photographing 10 points at 3,000 times magnification using Electron Backscatter Diffraction (EBSD).
  • SEM scanning electron microscope
  • the tensile strength is a value measured by taking a JIS No. 5 specimen in a direction perpendicular to the rolling direction at a width W / 4, and a tensile strength deviation (material deviation) [? TS (MPa)] is a tensile strength It represents the value obtained by subtracting the minimum value from the maximum value among the intensity values.
  • the formula (1) is represented by 1.9Al-3.4Ti, and each symbol represents a value (weight (%) / atomic weight of each element) obtained by dividing the weight (%) of each element by the atomic weight,
  • Example No. 2 Steel grade Rough rolling out bar plate edge temperature ( ⁇ ) Equation (3) (%) RSB (Bar) FSB (Bar) Finishing rolling temperature ( ⁇ ) Ar 3 (° C) Ar 3 -30 ⁇ ( ⁇ ) Mf ( ⁇ ⁇ ) Air cooling time (sec) ROT cooling rate (° C / sec) Coiling temperature ( ⁇ ) Inventory 1 Inventive Steel A 880 ⁇ 8.3X10 -7 189 216 811 760 730 282 3.4 225 165 Inventory 2 890 ⁇ 8.3X10 -7 200 230 815 3.5 215 170 Inventory 3 895 ⁇ 8.3X10 -7 181 210 814 3.2 210 169 Honorable 4 Invention steel B 900 ⁇ 8.3X10 -7 205 200 815 761 731 299 3.1 195 175 Inventory 5 882 ⁇ 8.3X10 -7 210 195 820 3.5 200 180 Inventory 6 892 ⁇ 8.3X10 -7 195 210 816 3.7 215
  • F Ferrite (ferrite)
  • M Martensite (martensite)
  • B Bainite (bainite) structure
  • FGS Ferrite Grain Size
  • YS Yield Strength
  • TS Tensile Strength
  • EL Elongation As shown in Tables 1, 2 and 3, Examples 1 to 7 satisfying the range of the present invention satisfy the target tensile strength (980 MPa or more) and elongation (10% or more) without slip of the molten steel in the production of the slab, and the edge surface quality and the surface quality Are also excellent. It is also understood that tensile strength and yield strength of Examples 1 to 7 are higher than those of Conventional Example 1. [
  • Inventive Example 7 was photographed using an optical microscope and a scanning electron microscope (SEM), the photographs of the optical microscope were shown in Fig. 1, and the photographs of the scanning electron microscope were shown in Fig. 1 and 2, it can be seen that the microstructure of the inventive example 7 is composed of ferrite (F) and M (martensite) in a phase and some bainite (B) is uniformly present .
  • SEM scanning electron microscope
  • the ferrite grain size distribution of Inventive Example 7 was measured using EBSD (Electron Backscatter Diffraction), and the results are shown in FIG. From Fig. 3, it can be seen that the crystal grains of 5 mu m or less are finely dispersed.
  • Example 7 The precipitate of Example 7 was photographed with a transmission electron microscope (TEM), and the photograph is shown in Fig. 4.
  • the left side of Fig. 4 is a 50,000 magnification photograph, and the right side is a 300,000 magnification photograph. From FIG. 4, it can be seen that the TiN, Ti (C, N) and the rounded TiC precipitates are uniformly distributed in the matrix.
  • the deposit number is 5-30 / ⁇ m 2 mainly distributed in the range
  • the average number of precipitates was 15 / ⁇ 2 .
  • the number of precipitates is obtained by quantifying the number of precipitates existing within a square of 1 mu m x 1 mu m in a tissue photograph obtained by making a sample by the carbon (carbon) replica method and photographing 50 places randomly at a magnification of 100,000 times with TEM .
  • the distribution of the precipitate size in Inventive Example 7 was observed. The results are shown in Fig. 6. As can be seen from Fig. 6, the precipitate size was mainly distributed in the range of 5 to 50 nm, and the average precipitate size was 20 nm.
  • the precipitate size was obtained by measuring the precipitate size using Image-Plus Pro software after preparing a sample by the carbon re-firing method and photographing 5 sheets of 50,000 times, 20 sheets of 100,000 sheets and 5 sheets of 300,000 sheets by TEM. TEM images were taken at different magnifications to precisely measure fine (less than 50 nm) precipitates and coarse (greater than 50 nm) precipitates.
  • FIG. 7 shows a surface photograph of the PO material obtained by pickling the hot-rolled steel sheet of Inventive Example 7, and FIG. 7 shows that the surface quality is excellent.
  • Comparative Examples 1, 2 and 3 do not satisfy the air cooling time, cooling rate, and coiling temperature proposed in the present invention, and do not satisfy the target microstructure and tensile strength.
  • Comparative Example 4 shows that the surface of the steel strip is hardly cooled because the surface of the steel strip is higher than the pressure of the FSB 1 and 2 columns proposed in the present invention and the finish rolling temperature is lowered rapidly so that the desired microstructure and tensile strength are not satisfied .
  • Comparative Examples 5 and 6 did not satisfy the RSB or FSB pressure suggested in the present invention, indicating that the surface quality is favorable.
  • Comparative Example 8 did not satisfy the C content shown in the present invention, and molten steel leakage occurred in the slab, Respectively.
  • the cause of casting interruption can be explained by the phase transformation behavior.
  • FIGS. 9 and 10 show state diagrams of Example 7 and Comparative Example 8 calculated using the Thermo-Calc-3.0.1 Console Mode (Database: TCFE6).
  • Thermo-Calc-3.0.1 Console Mode Database: TCFE6
  • the comparative example 7 did not satisfy the formula (3) proposed in the present invention, and the comparative example 10 did not satisfy the formula (1) and the formula (3) . These edge cracks are closely related to the precipitation behavior of AlN.
  • FIG. 11 and 12 show the precipitation behavior according to the temperature of the inventive example 7 and the comparative example 10 calculated using Thermo-Calc-3.0.1 (Database: TCFE6), respectively.
  • Fig. 13 shows the conventional example 1.
  • Fig. As can be seen from this result, in the case of Inventive Example 7 which satisfies the formula (1) and the Ti content proposed in the present invention, TiN precipitates near the solidification temperature of the molten steel and the amount of AlN precipitates is remarkably decreased, . However, in the case of Comparative Example 10 and Comparative Example 1 which do not satisfy the formula (1) and Ti content, AlN precipitates at a high temperature and a large amount of AlN is precipitated in the edge temperature range (780 to 1100 ° C) . Therefore, in order to secure surface and edge quality while ensuring high strength, it is preferable to precisely control it so as to satisfy equations (1) and (3).
  • Comparative Example 9 did not satisfy the target tensile strength because the Mn content was not satisfied in the present invention and Comparative Example 11 did not satisfy the edge temperature range of the rough rolling out side bar plate proposed in the present invention It can be seen that the arithmetic scale is generated in large quantity and the surface quality of the PO material is heat.
  • FIG. 14 shows a cross-sectional structural change of a nugget according to the welding current with respect to the inventive example 7.
  • the nugget diameter was defined as the straight line distance between the bond line (BL) and the bond line, as in the cross-section after cutting the nugget at 1/2.
  • the nugget diameter increases with the increase of the welding current, and it is found that the nugget has a healthy weld portion without defects such as cracks and pores.
  • FIG. 15 shows the hardness distribution of the resistance spot weld portion . The hardness was measured with a Vickers hardness gauge at 200 ⁇ intervals with 200 g load on the nugget diagonal.
  • the weld is divided into nose (Fusion, FZ), heat affected zone (HAZ), softening zone (SZ) near the base material and base material (BM).
  • the hardness of the molten portion has a value of 370 to 400 Hv, and the difference between the hardness of the base material and the minimum hardness of the softened portion is 100 Hv or less.
  • the softened part is a typical phenomenon observed in the AHSS weld part having a high martensite fraction. When the hardness of the softened part is too low, the softened part may be judged and the strength of the welded part may be low. Therefore, it is preferable that the difference between the hardness of the base material and the minimum hardness of the softened portion is 100 Hv or less.
  • the ductility ratio (CTS / TSS) is defined as the ratio of Tensile Shear Strength (TSS) to Cross Tensile Strength (CTS) and is used as a comprehensive index to judge the characteristics of the weld in the AHSS RSW .
  • TSS Tensile Shear Strength
  • CTS Cross Tensile Strength
  • the reason why 5t1 / 2 is used is that AHSS with a tensile strength of 980 MPa or higher has a high carbon equivalent (Ceq) and a rapid cooling rate specific to RSW, so that a martensite structure is formed in the molten portion and the fracture shear sensitivity to the interface , So it is indicated as 4t 1/2 that is based on the existing standard is not suitable.
  • the upper limit current is defined as a value obtained by subtracting 0.2 (kA) from the current generated by scattering. As can be seen from this result, IF break occurred at current satisfying 4t 1/2 , and POF break occurred at current more than this.
  • the appropriate welding range in RSW is a key index for evaluating weldability and can be defined as the interval between the upper limit current and the lower limit current.
  • the optimum welding range between the lower current limit and the upper current limit satisfying the minimum nugget diameter of 4t 1/2 is 2.6 (kA), and the minimum nugget diameter is 5t 1/2 , the optimum welding range is 1.8kA. It can be seen that the ductility ratio has more than 45% in the given appropriate welding current range regardless of the fracture.
  • Example 17 shows a SEM micrograph of a molten portion with respect to Example 7. It can be seen that a full lath martensite structure is uniformly present, which has a high carbon equivalent (Ceq) It is judged to be due to rapid cooling rate.

Abstract

La présente invention concerne une tôle d'acier laminée à chaud de haute résistance perfectionnée, ayant une résistance à la traction de 980 MPa, et son procédé de fabrication, le procédé assurant une qualité de surface, une aptitude au traitement et une soudabilité améliorées ainsi qu'un écart significativement réduit du matériau dans les directions de la largeur et de la longueur de la tôle d'acier grâce à un mode de laminage sans fin dans un processus de coulée continue-laminage direct. Selon un aspect préféré, l'invention concerne une tôle d'acier laminée à chaud à haute résistance perfectionnée, ayant une microstructure, et son procédé de fabrication, la tôle d'acier laminée à chaud à haute résistance perfectionnée étant fabriquée au moyen d'un procédé coulée continue-laminage direct et comprenant, en % en poids, 0,030-0,085 % de C, 1,8-3,0 % de Mn, 0,03-1,0 % de Si, 0,005 à 0,05 % de P, 0,01 % ou moins de S, 0,2 à 2,0 % de Cr, 0,01 à 0,07 % d'Al, 0,005 à 0,10 % de Ti, 0,0005 à 0,0050 % de B, 0,001 à 0,010 % de N, le reste étant du Fe, et les inévitables impuretés et, dans une fraction de surface, 40 à 60 % de ferrite, 30 à 50 % de martensite et 10 à 20 % de bainite.
PCT/KR2018/007721 2017-07-07 2018-07-06 Tôle d'acier laminée à chaud de haute résistance perfectionnée et son procédé de fabrication WO2019009677A1 (fr)

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CN114086073A (zh) * 2021-11-19 2022-02-25 安徽工业大学 一种热轧高强结构钢的生产方法

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