EP4159885A1 - 780 kaltgewalzter und geglühter zweiphasenstahl der klasse mpa und herstellungsverfahren dafür - Google Patents

780 kaltgewalzter und geglühter zweiphasenstahl der klasse mpa und herstellungsverfahren dafür Download PDF

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EP4159885A1
EP4159885A1 EP21813104.3A EP21813104A EP4159885A1 EP 4159885 A1 EP4159885 A1 EP 4159885A1 EP 21813104 A EP21813104 A EP 21813104A EP 4159885 A1 EP4159885 A1 EP 4159885A1
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cold
rolled
phase steel
annealed dual
dual
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French (fr)
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EP4159885A4 (de
EP4159885A9 (de
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Wei Li
Xiaodong Zhu
Peng XUE
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Baoshan Iron and Steel Co Ltd
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Baoshan Iron and Steel Co Ltd
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    • C—CHEMISTRY; METALLURGY
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    • 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
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
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    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
    • B21C47/02—Winding-up or coiling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • 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
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    • 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
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    • 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
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
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    • 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
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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
    • 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 disclosure relates to a metallic material and a method for manufacturing the same, particularly to a cold-rolled and annealed dual-phase steel and a method for manufacturing the same.
  • High-strength dual-phase steel has good mechanical properties and usability, and can be effectively used to produce vehicle structural parts.
  • ultra-high strength steel is economical and has better performances.
  • 780 DP steel is still the mainstream steel in applications. It accounts for 60% of the total amount of DP steel, and it is widely used for various types of structural members and safety members.
  • the main concerns in the development of dual-phase steel in the future must be low cost and high performances in combination.
  • Canadian Patent Application No. CA2526488 published on December 2, 2004 and entitled "A COLD-ROLLED STEEL SHEET HAVING A TENSILE STRENGTH OF 780 MPA OR MORE, AN EXCELLENT LOCAL FORMABILITY AND A SUPPRESSED INCREASE IN WELD HARDNESS” discloses a cold-rolled steel sheet having a chemical composition of: C: 0.05-0.09%; Si: 0.4-1.3%; Mn: 2.5-3.2%; optional Mo: 0.05-0.5% or Ni: 0.05-2%; P: 0.001-0.05%; S ⁇ 0.08 ⁇ Ti-3.43 ⁇ N+0.004; N ⁇ 0.006%; Al: 0.005-0.10%; Ti: 0.001-0.045%; optional Nb ⁇ 0.04% or B: 0.0002-0.0015%; optional Ca for treatment, with the balance of Fe and unavoidable impurities.
  • United States Patent Publication No. US20050167007 published on August 4, 2005 discloses a method for manufacturing a high-strength steel sheet comprising the following chemical composition: 0.05-0.13% C, 0.5-2.5% Si, 0.5-3.5% Mn, 0.05-1% Cr, 0.05-0.6% Mo, ⁇ 0.1% Al, ⁇ 0.005% S, ⁇ 0.01% N, ⁇ 0.03% P, with addition of 0.005-0.05% Ti or 0.005-0.05% Nb or 0.005-0.2% V.
  • the steel is hot rolled at a temperature equal to or higher than Ar3, coiled at 450-700 °C, annealed, quenched from 700-600 °C by cooling at a cooling rate of 100 °C/s, and then tempered at 180-450 °C. Finally, a high-strength steel having a tensile strength of 780 Mpa and a hole expansion rate of higher than 50% is obtained.
  • the main problem of this steel is that the total amount of alloy is too high and the Si content is high, which is detrimental to the weldability or phosphatability of the steel.
  • the 780 Mpa dual-phase steels designed according to some of the existing patent technologies exhibit good formability, they have either high contents of C and Si, or high contents of alloy elements such as Cr, Ni, and Mo. This is detrimental to the weldability, surface quality or phosphatability of the steels, and the cost is also high.
  • the hole expansion rate is very high and the bendability is good, the yield ratio is high, and the stamping performance is degraded.
  • One of the objects of the present disclosure is to provide an economical 780 MPa grade cold-rolled and annealed dual-phase steel.
  • the resulting steel plate has a strength of 780MPa grade with no addition of Mo and Cr, and a fine and uniform martensite + ferrite dual-phase structure is obtained to ensure excellent performances of elongation and cold bending, so that the steel has good formability.
  • the cold-rolled and annealed dual-phase steel has a yield strength of ⁇ 420MPa; a tensile strength of >780MPa; an elongation at break with A50 gauge length of ⁇ 18%; a 90-degree cold bending parameter R/t ⁇ 1, where R represents bending radius in mm, and t represents plate thickness in mm.
  • the present disclosure provides a 780MPa grade cold-rolled and annealed dual-phase steel having a matrix structure of fine and uniform martensite + ferrite, wherein the cold-rolled and annealed dual-phase steel comprises the following chemical elements in mass percentages, in addition to Fe:
  • the cold-rolled and annealed dual-phase steel in the present disclosure comprises the following chemical elements in mass percentages: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and a balance of Fe and other unavoidable impurities.
  • a composition system with C and Mn as the dominant additive elements is designed for the composition of the cold-rolled and annealed dual-phase steel according to the present disclosure, so as to ensure that the cold-rolled and annealed dual-phase steel can reach a strength of 780 MPa grade.
  • the absence of precious alloy elements such as Mo and Cr can effectively guarantee the economic efficiency.
  • the addition of Nb and Ti in trace amounts can achieve the effect of inhibiting growth of austenite grains, and can effectively refine the grains.
  • the strength of the hot rolled coil is not too high, which can guarantee the processability in cold rolling.
  • C In the cold-rolled and annealed dual-phase steel according to the present disclosure, the addition of the C element can improve the strength of the steel and the hardness of martensite. If the mass percentage of C in the steel is lower than 0.1%, the strength of the steel plate will be affected, and it is detrimental to formation and stability of austenite. If the mass percentage of C in the steel is higher than 0.13%, the hardness of martensitic will be too high, and the grain size will be large, which is detrimental to the formability of the steel plate. At the same time, an unduly high carbon equivalent is detrimental to welding in use. Therefore, in the cold-rolled and annealed dual-phase steel according to the present disclosure, the mass percentage of C is controlled at 0.1%-0.13%.
  • the mass percentage of C may be controlled at 0.11%-0.125%.
  • the addition of the Si element to the steel can improve hardenability.
  • the solid dissolved Si in the steel may have an effect on the interaction of dislocations, thereby increasing the work hardening rate. This may increase the elongation of the dual-phase steel suitably, which is beneficial to obtain better formability.
  • the mass percentage of Si in the steel is controlled at 0.4%-0.8%.
  • the mass percentage of Si may be controlled at 0.5%-0.7%.
  • Mn In the cold-rolled and annealed dual-phase steel according to the present disclosure, the addition of the Mn element is beneficial to improve the hardenability of the steel, and can effectively improve the strength of the steel plate. However, it should be noted that when the mass percentage of Mn in the steel is lower than 1.65%, the strength of the steel plate will be insufficient; when the mass percentage of Mn in the steel is higher than 1.9%, the strength of the steel plate will be too high to reduce its formability. Therefore, in the cold-rolled and annealed dual-phase steel according to the present disclosure, the mass percentage of Mn is controlled at 1.65%-1.9%.
  • the mass percentage of Mn may be controlled at 1.7%-1.8%.
  • Al In the cold-rolled and annealed dual-phase steel according to the present disclosure, the addition of Al may have the effect of removing oxygen and refining grains. Therefore, in the cold-rolled and annealed dual-phase steel according to the present disclosure, the mass percentage of Al is controlled at 0.01%-0.05%.
  • the mass percentage of Al may be controlled at 0.015-0.045%.
  • the Nb element is an important element for grain refinement.
  • a strain-induced precipitation phase can be formed in the controlled rolling process.
  • the strain-induced precipitation phase can significantly reduce the recrystallization temperature of deformed austenite by means of the action of particle pinning and subgrain boundaries, provide nucleation particles, and thus have a significant effect of refining grains.
  • the mass percentage of Nb is controlled at 0.01-0.03%.
  • the mass percentage of Nb may be controlled at 0.015-0.025%.
  • the strong carbide forming element Ti added to the cold-rolled and annealed dual-phase steel according to the present disclosure also exhibits a strong effect of inhibiting growth of austenite grains at high temperatures. At the same time, the addition of Ti helps to refine grains. Therefore, in the cold-rolled and annealed dual-phase steel according to the present disclosure, the mass percentage of Ti is controlled at 0.01-0.03%.
  • the mass percentage of Ti may be controlled at 0.015-0.025%.
  • the mass percentage contents of Nb and Ti in the cold-rolled and annealed dual-phase steel may further satisfy the following formula: Nb%+Ti% ⁇ 3 ⁇ 0.047%, preferably ⁇ 0.06%.
  • Nb and Ti each represent the mass percentage content of the corresponding element, that is, the value in front of the percent sign in the formula. In some embodiments, 0.047% ⁇ Nb%+Ti% ⁇ 3 ⁇ 0.10%; preferably, 0.06% ⁇ Nb%+Ti% ⁇ 3 ⁇ 0.10%.
  • the mass percentage contents of the chemical elements satisfy at least one of the following:
  • the unavoidable impurities include the P, S and N elements, and the contents thereof are controlled to be at least one of the following: P ⁇ 0.015%, S ⁇ 0.003%, N ⁇ 0.005%.
  • the P, N and S elements are all unavoidable impurity elements in the steel. It's better to lower the contents of the P, N and S elements in the steel as far as possible. MnS formed from S seriously affects the formability, and N tends to incur cracks or bubbles on the surface of the slab. Therefore, in the cold-rolled and annealed dual-phase steel according to the present disclosure, the mass percentage of P is controlled at P ⁇ 0.015%; the mass percentage of S is controlled at S ⁇ 0.003%; and the mass percentage of N is controlled at N ⁇ 0.005%.
  • the phase proportion (by volume) of martensite is >55%.
  • the grain diameter of martensite is not greater than 5 microns, and the grain diameter of ferrite is not greater than 5 microns.
  • the performances of the cold-rolled and annealed dual-phase steel according to the present disclosure satisfy at least one of the following: yield strength ⁇ 420 MPa, preferably ⁇ 430 MPa; tensile strength>780 MPa, preferably ⁇ 800 MPa; elongation at break with A50 gauge length ⁇ 18%; a 90-degree cold bending parameter R/t ⁇ 1, where R represents bending radius in mm, t represents plate thickness in mm.
  • the performances of the cold-rolled and annealed dual-phase steel according to the present disclosure satisfy the following: yield strength ⁇ 420 MPa, preferably ⁇ 430 MPa; tensile strength>780 MPa, preferably ⁇ 800 MPa; elongation at break with A50 gauge length ⁇ 18%; 90 degree cold bending parameter R/t ⁇ 1, where R represents bending radius in mm, t represents plate thickness in mm.
  • the yield ratio of the cold-rolled and annealed dual-phase steel according to the present disclosure is 0.53-0.57.
  • another object of the present disclosure is to provide a method for manufacturing a cold-rolled and annealed dual-phase steel.
  • the cold-rolled and annealed dual-phase steel made by the manufacturing method has the characteristics of high strength, excellent elongation and cold bending performance. It has a yield strength of ⁇ 420MPa; a tensile strength of >780MPa; an elongation at break with A50 gauge length of ⁇ 18%; a 90-degree cold bending parameter R/t ⁇ 1, where R represents bending radius in mm, and t represents plate thickness in mm.
  • the present disclosure proposes a method for manufacturing the above cold-rolled and annealed dual-phase steel, comprising steps of:
  • the reason for controlling the annealing soaking temperature at 770-820 °C is as follows: when the annealing soaking temperature is lower than 770 °C, the steel having a strength of 780 MPa grade cannot be obtained; while if the annealing soaking temperature is higher than 820 °C, the grain size will be large, which will greatly degrade the formability. Therefore, controlling the annealing soaking temperature at 770-820 °C can ensure obtainment of both the tensile strength of 780MPa and the small grain size, so that the cold-rolled and annealed dual-phase steel has better formability.
  • the annealing soaking temperature may be controlled in the range of 790-810 °C in order to obtain better implementation effects, i.e. to obtain a smaller grain size, moderate mechanical properties of the steel obtained, and better formability.
  • step (2) the slab is first heated to 1160-1220 °C, preferably 1165-1215 °C; held for 0.6 hours or longer, preferably 0.6-1.5 hours; hot rolled at a temperature of 850-900 °C; rapidly cooled at a rate of 30-80 °C/s after the rolling; coiled with the coiling temperature being controlled at 500-600 °C, preferably 520-600 °C; and air cooled after the coiling.
  • step (3) a cold rolling reduction rate is controlled at 50-70%.
  • a tempering temperature is controlled at 200-270 °C, and a tempering time is 100-400 s, preferably 150-400 s.
  • step (6) a temper rolling reduction rate is controlled at ⁇ 0.3%.
  • the annealing soaking temperature is 790-810 °C.
  • the alloy chemical composition in the cold-rolled and annealed dual-phase steel is designed reasonably, so that a steel plate having a strength of more than 780MPa grade and a martensite + ferrite dual-phase structure is obtained without addition of Mo and Cr.
  • the steel plate has a yield strength of ⁇ 420MPa; a tensile strength of >780MPa; an elongation at break with A50 gauge length of ⁇ 18%; a 90-degree cold bending parameter R/t ⁇ 1, where R represents bending radius in mm, and t represents plate thickness in mm. While good economy is achieved, the steel plate has the characteristics of high strength, excellent elongation and cold bending performance.
  • the cold-rolled and annealed dual-phase steel obtained by the manufacturing method according to the present disclosure not only has good economy, but also has the characteristics of high strength, excellent elongation and cold bending performance.
  • Figure 1 shows the structure of the cold-rolled and annealed dual-phase steel of Example 1.
  • Table 1 lists the mass percentages of various chemical elements in the steel grades corresponding to the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.
  • Table 1 (wt%, the balance is Fe and unavoidable impurities other than P, S and N) Steel grade Chemical elements C Si Mn Al P S N Nb Ti Nb%+Ti% ⁇ 3 Ex. 1 A 0.103 0.45 1.74 0.012 0.014 0.0015 0.0035 0.012 0.021 0.075 Ex. 2 B 0.106 0.52 1.68 0.032 0.013 0.0016 0.0033 0.017 0.015 0.062 Ex.
  • Tables 2-1 and 2-2 list the specific process parameters for the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.
  • Step (4) Step (6) Annealing soaking temperature (°C) Annealing time (s) Cooling rate (°C/s) Starting temperature of rapid cooling (°C) Rapid cooling rate (°C/s) Ending temperature of rapid cooling (°C) Temperin g temperatu re (°C) Temperin g time (s) Temper rolling reduction rate (%) Ex. 1 815 150 5 705 55 200 200 220 0.3 Ex. 2 785 90 4 715 44 250 250 300 0.2 Ex.
  • Table 3 lists the performance test results for the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.
  • Table 3 No. Yield strength (MPa) Tensile strength (MPa) Elongation at break A 50 (%) 90° bending radius R (mm) Plate thickness t (mm) R/t Ex. 1 454 800 22.3 1.0 1.1 0.91 Ex. 2 435 812 21.5 1.0 1.1 0.91 Ex. 3 474 856 19.5 1.0 1.1 0.91 Ex. 4 449 832 20.5 1.0 1.2 0.83 Ex. 5 458 827 208 1.0 1.2 0.83 Ex. 6 476 872 19.7 1.0 1.2 0.83 Ex. 7 489 884 18.4 1.0 1.1 0.91 Comp. Ex.
  • Examples 1-7 meeting the control requirements of the design specification according to the present disclosure have excellent performances, including yield strength ⁇ 420 MPa; tensile strength>780 MPa; elongation at break with A50 gauge length ⁇ 18%; a 90-degree cold bending parameter R/t ⁇ 1 (R represents bending radius in mm, t represents plate thickness in mm).
  • R/t ⁇ 1 a 90-degree cold bending parameter
  • the various performances of the cold-rolled and annealed dual-phase steels of the various Examples are quite excellent. With no addition of precious alloy elements such as Mo and Cr, the steels achieve a tensile strength of greater than 780 MPa, and exhibit good elongation and superior cold bending performance.

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