Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
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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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
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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
  • C21D8/0226—Hot rolling
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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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0242—Flattening; Dressing; Flexing
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—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
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • 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
  • 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
  • C22C38/14—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
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a steel material and a manufacturing method thereof, in particular to a cold-rolled steel sheet and a manufacturing method thereof.
• Ultra-high-strength steel has good mechanical properties and usability. It can be used to manufacture automotive structural parts and provide lightweight components, thereby reducing vehicle weight effectively. In the current automotive industry, for the manufacture of vehicles in practice, higher strength steel sheets are required to be used due to the need for weight reduction and safety. Ultra-high-strength steel with a tensile strength of 1000 MPa or higher is quite promising in weight reduction and safety performance. It can be used in the manufacture of safety parts, reinforcement parts and structural parts, and has good promotion prospects.
• high-strength steel with a tensile strength of 1000MPa or higher has an inherent characteristic of stress corrosion cracking (delayed cracking).
• This high-strength steel sheet is prone to slow cracking under the action of stress and a corrosive medium.
• the delayed cracking has caused considerable trouble for the application of high-strength steel, and has greatly limited the application of ultra-high-strength steel.
• delayed cracking means that a part does not crack when it's manufactured, but as time goes by, under the dual effects of stress and a corrosive medium, stress corrosion cracking occurs, which eventually leads to failure of the part and loss of safety protection.
• hydrogen plays a role in promoting crack initiation and propagation.
• the higher the strength of the steel material the more serious the tendency to delayed cracking becomes. Delayed cracking is the biggest risk in the application of advanced high-strength steel.
• the cold-rolled steel sheet needs to be cooled from Ac3 at a cooling rate of 20 °C/s or higher (gas cooling) to the range of from Ms point to Ms point + 200°C, held for 0.1 to 60 s, and then cooled to 100 °C or lower at a cooling rate of 100 °C/s or higher (water cooling) to obtain the high-strength steel with a tensile strength of 1320 MPa or higher, wherein the flatness of the steel sheet is 10 mm or less.
• the chemical elements of the steel sheet and their contents are: C 0.14%-0.35%, Mn 1.5%-2.0%, Si 0.6%-1.0%, P ⁇ 0.015%, S ⁇ 0.002%, Nb 0.01%-0.06%, B 0.0005%-0.0040%, La 0.001%-0.5%, and a balance of Fe and unavoidable impurities.
• the heat treatment process system adopted is as follows: austenitizing temperature is 880-940 °C; holding time is 0.5-5 hours, followed by water quenching; tempering temperature is 190-250 °C; and holding time is 1-15 hours.
• the designed steel sheet has excellent mechanical properties, including a tensile strength of 1200-1400 MPa, a yield strength of 1000-1300 MPa, and an elongation of 6-15%. It features low production cost, and capability of being industrially produced with a thickness of 5-25 mm.
• the steel designed by this technical solution has a tensile strength of 1340 MPa or higher and an elongation after fracture of less than 12%. Its "U"-shaped notch impact absorption energy is greater than 72 J. It has a high strength, few quenching cracks and internal cracks, and a long service life.
• One of the objects of the present disclosure is to provide a novel 1300 MPa or higher grade cold-rolled steel sheet.
• a reasonable chemical composition design and a corresponding manufacturing process are utilized for this 1300 MPa or higher grade cold-rolled steel sheet. While having ultra-high strength, it also has excellent delayed cracking resistance and bending performance.
• the preset stress is greater than or equal to 1.05 times the tensile strength, the cold-rolled steel sheet can be immersed in 1 mol/L hydrochloric acid for 300 hours or longer without delayed cracking. It is particularly suitable for manufacture of automotive safety structural parts, and has good prospects for promotion and application.
• the present disclosure provides a 1300 MPa or higher grade cold-rolled steel sheet comprising Fe and unavoidable impurity elements, and further comprising the following chemical elements in mass percentages:
• the mass percentages of the chemical elements are as follows:
• C In the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, the addition of the C element can not only improve the strength of the steel, but also improve the hardness of martensite to ensure occurrence of martensitic transformation.
• the mass percentage of the C element in the steel is lower than 0.1%, the strength of the steel sheet will be affected, and it's disadvantageous with respect to the amount and stability of the austenite formed; and when the mass percentage of the C element in the steel is higher than 0.30%, it will readily cause the martensite hardness to be too high and the grains to be coarse, which is not conducive to the forming performance of the steel sheet. Therefore, in view of the influence of the C content on the properties of the steel, in the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, the mass percentage of the C element is controlled in the range of 0.10% - 0.30%.
• the addition of the Si element can improve the hardenability of the steel.
• the solid-dissolved Si in the steel may influence the interaction of the dislocations, increase the work hardening rate, and improve the elongation appropriately, which is beneficial for the steel to acquire better formability.
• the mass percentage of the Si element is controlled in the range of 0.1% - 0.5%.
• Mn In the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, the addition of the Mn element can not only improve the hardenability of the steel, but also effectively improve the strength of the steel sheet.
• the mass percentage of Mn in the steel is selected to be 0.8% - 2.5% for the reason that: when the mass percentage of Mn in the steel is lower than 0.8%, the hardenability of the steel prepared is insufficient, a sufficient amount of martensite cannot be generated during the annealing process, and the strength of the steel sheet is insufficient; and when the mass percentage of Mn in the steel is higher than 2.5%, the carbon equivalent will increase significantly, which has a negative impact on both the welding performance and the delayed cracking resistance of the steel. Therefore, in view of the influence of the Mn content on the properties of the steel, in the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, the mass percentage of the Mn element is controlled in the range of 0.8% - 2.5%.
• the Al element added to the steel in a proper amount can play a role in deoxidation and grain refinement. Therefore, in order to bring into play the beneficial effects of the Al element, in the present disclosure, the mass percentage of the Al element is controlled in the range of 0.01% - 0.03%.
• B In the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, B is an element that can significantly improve the hardenability of the steel.
• the addition of the B element can promote the formation of martensite and guarantee the strength of the martensitic steel.
• the content of the B element in the steel should not be too high.
• a "boron phase” will precipitate at the grain boundary, which will increase the grain boundary energy level.
• the "boron phase” will also serve as a core of a new phase to increase the nucleation rate and cause the hardenability of the steel to decrease. Therefore, in view of the influence of the B content on the properties of the steel, in the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, the mass percentage of the B element is controlled in the range of 0.001-0.003%.
• Ti In the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure, Ti added as a strong carbide-forming element will show a strong effect of inhibiting the growth of austenite grains at high temperatures. At the same time, the addition of the Ti element to the steel also helps to refine the grains. Therefore, in order to bring into play the beneficial effects of the Ti element, in the present disclosure, the mass percentage of the Ti element is controlled in the range of 0 - 0.05%. Preferably, the mass percentage of the Ti element is controlled in the range of 0.01 - 0.05%.
• the inventors not only control the mass percentages of the individual chemical elements, but also further control the mass percentages of the C and Mn elements in the steel to satisfy: C+Mn/6 ⁇ 0.35%.
• C+Mn/6 ⁇ 0.35% Preferably, 0.35% ⁇ C+Mn/6 ⁇ 0.60%; more preferably, 0.37% ⁇ C+Mn/6 ⁇ 0.56%.
• the P element, the S element and the N element are all impurity elements in the steel.
• the contents of the impurity elements in the steel should be minimized. Therefore, except there is a special need, the content of the P element in the steel should be reduced as much as possible, and particularly the mass percentage of P should be controlled to be P ⁇ 0.015%.
• the mass percentage of the S element in the steel is strictly controlled to satisfy S ⁇ 0.003%.
• the impurity element N can easily cause cracks or bubbles to form on the surface of the slab, in the present disclosure, the mass percentage of the N element is controlled to satisfy N ⁇ 0.006%.
• microstructure of the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure is retained austenite + fine massive tempered martensite + bainite.
• the volumetric phase fraction of the tempered martensite is ⁇ 55%, and the volumetric phase fraction of the bainite is greater than 0 and ⁇ 15%.
• the volumetric phase fraction of the tempered martensite is 55 - 90% or 65 - 90%.
• the volumetric phase fraction of the bainite is 5-15%.
• the diameter of the tempered martensite is not greater than 10 microns; preferably, the diameter of the tempered martensite is 4 - 9 microns.
• the composition of the steel is designed to be a composition system mainly based on C+Mn+B.
• the volumetric fraction of the martensite can be ensured to be greater than 55%.
• the cold-rolled steel sheet acquires a microstructure of retained austenite + fine massive tempered martensite (with the diameter of the massive martensite being not more than 10 microns) + bainite.
• the stress and hardness are reduced, and at the same time, fine dispersed precipitates that can serve as hydrogen traps can be produced inside. All of these are factors that are conducive to improving the delayed cracking performance.
• the acquisition of the retained austenite is not only conducive to delayed cracking, but also conducive to improving the forming performance of the cold-rolled steel sheet.
• the properties of the 1300 MPa or higher grade cold-rolled steel sheet described in the present disclosure satisfy:
• the 90-degree cold bending performance characterization parameter R/t limit value is 2.0-2.5; when the tensile strength is greater than 1400 MPa and ⁇ 1500 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is 2.5-3; when the tensile strength is greater than 1500 MPa and ⁇ 1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is 3.0-3.5; when the tensile strength is greater than 1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is 3.5-4.
• the preset stress when the preset stress is greater than or equal to 1.1 times the tensile strength, no delayed cracking occurs after immersion in 1 mol/L hydrochloric acid for 300 hours or longer.
• the preset stress when the preset stress is greater than or equal to 1.15 times the tensile strength, no delayed cracking occurs after immersion in 1 mol/L hydrochloric acid for 300 hours or longer.
• the preset stress when the preset stress is greater than or equal to 1.2 times the tensile strength, no delayed cracking occurs after immersion in 1 mol/L hydrochloric acid for 300 hours or longer.
• the acquisition of the combination of retained austenite + fine massive tempered martensite + bainite determines the superior forming performance of the 1300 MPa or higher grade cold-rolled steel sheet designed according to the present disclosure.
• the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 2.5; when the tensile strength is 1401-1500 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 3; when the tensile strength is 1501-1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 3.5; when the tensile strength is greater than 1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 4.
• another object of the present disclosure is to provide a method for manufacturing the above 1300 MPa or higher grade cold-rolled steel sheet, which optimizes the design of the manufacturing process.
• the cold-rolled steel sheet manufactured using this manufacturing method has ultra-high strength as well as excellent delayed cracking resistance and bending performance.
• the present disclosure proposes a method for manufacturing the above 1300 MPa or higher grade cold-rolled steel sheet, comprising the following steps:
• the acquisition of bainite is one of the characteristics of the present disclosure.
• the steel can first acquire part of the bainite to ensure that the martensite generated subsequently forms fine dispersed nuclei around the bainite and does not grow violently, thereby eventually forming fine massive martensite. Fine massive tempered martensite with a diameter not greater than 10 microns can be obtained eventually.
• another characteristic of the present disclosure is that two tempering processes are designed and optimized in the manufacturing method. After the first continuous tempering process is completed and after the temper rolling is completed, a discontinuous secondary tempering process is further performed.
• the purpose of this design is to temper the martensite structure and enrich the untransformed austenite with carbon, so as to obtain the final structure of retained austenite + fine massive tempered martensite + bainite after cooling.
• the tempering temperature is specifically controlled to be 400-550 °C, and the tempering time is controlled to be 10-300 s, because this process determines the final morphology and size of the martensite.
• the tempering temperature and tempering time for each specific component need to be set specifically according to the dynamic CCT curve to ensure that the fraction of the resulting bainite is 15% or less, which will not have a significant impact on the strength of the steel.
• the tempering temperature is specifically controlled to be 180-260 °C, and the tempering time is 0.5-6 h.
• This process is independent of the aforementioned steps (4) and (5), and is therefore called a discontinuous tempering process. It's essentially a discontinuous low-temperature over-aging tempering process, which can be implemented using a bell-type furnace.
• the discontinuous tempering process the martensite structure can be tempered, and the untransformed austenite can be enriched with carbon, so that the final structure of retained austenite + fine tempered martensite + bainite can be obtained after cooling.
• the discontinuous tempering process of step (7) also needs to be designed reasonably according to the specific composition.
• the tempering temperature is too high and/or the tempering time is too long, it is likely to reduce the strength of the steel or result in a serious yield platform in the material, affecting the stamping performance.
• the tempering temperature is too low and/or the tempering time is too short, the martensite cannot be tempered significantly, so that a sufficient amount of retained austenite cannot be obtained, and the forming performance cannot be improved. Therefore, in order to guarantee the properties of the steel, in the present disclosure, the tempering temperature in the discontinuous tempering process is specifically controlled to be 180-260 °C, and the tempering time is 0.5-6 h.
• step (2) the cast blank obtained in step (1) is first heated to 1100-1250°C and held for 0.3 hours or longer (e.g. 0.3 to 2 hours), and then hot rolled at a temperature of Ar3 (austenite transformation temperature) or higher. After rolling, the blank is rapidly cooled at a rate of 30-80 °C/s, and the coiling temperature is controlled to be 530-600 °C.
• Ar3 austenite transformation temperature
• step (2) the hot rolling temperature does not exceed 920 °C.
• step (3) the cold rolling reduction rate is controlled to be 45-65%.
• step (4) the annealing soaking temperature is controlled to be 830-870 °C, and the holding time is 30-150 s, followed by cooling to 730-780 °C at a cooling rate of 5-15 °C/s, and then further cooling to the continuous tempering temperature at a rate of 50-700 °C/s.
• the annealing soaking temperature is limited to 830-870 °C, and the holding time is 30-150 s, because what is to be achieved is soaking annealing at a temperature for complete austenitization.
• the annealing soaking temperature used in step (4) is lower than 830 °C and the holding time is less than 30 seconds, sufficient tensile strength cannot be obtained; and when the annealing soaking temperature used is higher than 870 °C and the holding time is more than 150 seconds, the forming performance of the steel will be degraded greatly.
• the annealing soaking temperature can be preferably controlled in the range of 850-860 °C. This can both ensure complete austenitization and prevent the resulting grains from coarsening, thereby providing better forming performance.
• step (4) the annealing soaking temperature is controlled to be 850-860 °C.
• the tempering temperature is 430-550 °C.
• step (5) the tempering time is 50-300 s.
• step (5) preferably, the cooling rate is 30-50 °C/s.
• step (6) the temper rolling elongation is controlled to be 0-0.3%.
• the 1300 MPa or higher grade cold-rolled steel sheet of the present disclosure and the method for manufacturing the same have the following advantages and beneficial effects:
• a novel 1300 MPa or higher grade cold-rolled steel sheet and a method for manufacturing the same have been developed according to the present disclosure.
• the 1300 MPa or higher grade cold-rolled steel sheet has excellent delayed cracking resistance.
• the preset stress is greater than or equal to 1.05 times the tensile strength, it can be immersed in 1mol/L hydrochloric acid for 300 hours or longer without delayed cracking.
• the microstructure of retained austenite + fine massive tempered martensite + bainite possessed by this cold-rolled steel sheet determines directly that the cold-rolled steel sheet designed according to the present disclosure has good forming performance.
• the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 2.5; when the tensile strength is greater than 1400 MPa and ⁇ 1500 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 3; when the tensile strength is greater than 1500 MPa and ⁇ 1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 3.5; when the tensile strength is greater than 1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value is ⁇ 4, wherein R represents the bending radius, and t represents the plate thickness.
• the cold-rolled steel sheet designed according to the present disclosure has not only ultra-high strength, but also excellent delayed cracking resistance and bending forming performance. It can be used effectively to prepare automotive parts and be used in the automotive industry. It has good promotion prospects and application value.
• Table 1 lists the mass percentages of the various chemical elements designed for the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18.
• Table 1. (wt%, the balance is Fe and other unavoidable impurities besides P, S and N) No. Chemistry elements C+ Mn/6 (wt%) C (wt%) Si (wt%) Mn (wt%) Al (wt%) P (wt%) S (wt%) N (wt%) Ti (wt%) B (wt%) Ex. 1 0.107 0.13 2.43 0.022 0.013 0.0027 0.0056 0.012 0.0012 0.512 Ex. 2 0.106 0.13 2.42 0.021 0.013 0.0028 0.0054 0.011 0.0012 0.509 Ex.
• the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18 described in the present disclosure were each prepared by the following steps:
• Table 2-1 and Table 2-2 list the specific process parameters for the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18 in the above process steps.
• the finished 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18 obtained by the above process steps (1)-(7) were sampled respectively, and the microstructure of the steel sheet of each Example was observed and analyzed. It was found that the microstructure of the cold-rolled steel sheet of each Example was retained austenite + fine massive tempered martensite + bainite. In this disclosure, the microstructure was observed using a ZEISS Axio Imager M2m optical microscope. In addition, the details of the nano-precipitates and microstructure were further observed and analyzed by spherical aberration-corrected field emission transmission electron microscopy (TEM; model JEOL ARM-200F) with an operating accelerating voltage of 200 kV.
• TEM spherical aberration-corrected field emission transmission electron microscopy
• the volumetric phase fraction of the tempered martensite in the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18 was in the range of 68-91%, the volumetric phase fraction of the bainite was in the range of 5-14%, and the diameter of the tempered martensite was in the range of 4.3-8.7 microns.
• Tensile test The test was performed according to GB/T 228 (Metallic materials - Tensile testing - Part 1: Method of test at room temperature) to measure the yield strength, tensile strength and elongation of the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18.
• the bending performance of the cold-rolled steel sheet of each Example was characterized by the 90-degree cold bending performance characterization parameter R/t limit value, wherein the plate thickness t was fixed, and the bending radius R ensuring bending without cracking was variable.
• the bending radius R ensuring bending without cracking was a minimum, an R/t limit value was obtained.
• a larger 90-degree cold bending performance characterization parameter R/t limit value indicates a worse bending ability; whereas a smaller 90-degree cold bending performance characterization parameter R/t limit value indicates a better bending ability.
• Table 4 lists the mechanical property test results of the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18. Table 4. No. Yield strength (MPa) Tensile strength (MPa) Elongation (%) 90-Degree cold bending performance characterization parameter R/t limit value Ex. 1 1187 1434 7.8 2.5 Ex. 2 1179 1422 7.5 2.5 Ex. 3 1190 1438 7.7 2.5 Ex. 4 1453 1613 6.8 3.0 Ex. 5 1491 1626 6.9 3.0 Ex. 6 1487 1593 7.1 3.0 Ex. 7 1145 1336 10.1 2.0 Ex. 8 1137 1344 9.2 2.0 Ex. 9 1128 1322 9.7 2.0 Ex. 10 1501 1732 5.3 3.5 Ex. 11 1503 1738 6.5 3.5 Ex.
• the 1300 MPa or higher grade cold-rolled steel sheets of Examples 1-18 described in the present disclosure had both ultra-high strength and good cold bending deformability. They had a yield strength in the range of 1128-1503 MPa, a tensile strength in the range of 1321-1738 MPa, and an elongation in the range of 6.1% - 10.1%.
• the 90-degree cold bending performance characterization parameter R/t limit value was ⁇ 2.5; when the tensile strength was 1401-1500 MPa, the 90-degree cold bending performance characterization parameter R/t limit value was ⁇ 3; when the tensile strength was 1501-1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value was ⁇ 3.5; when the tensile strength was greater than 1650 MPa, the 90-degree cold bending performance characterization parameter R/t limit value was ⁇ 4. This shows that the cold-rolled steel sheets of Examples 1-18 had excellent bending deformability while having ultra-high strength.
• the cold-rolled steel sheets of Examples 1-18 prepared according to the present disclosure not only had the above-mentioned excellent mechanical properties, but also had excellent delayed cracking resistance.
• the inventors sampled the steel sheet of each Example again to perform an acid soaking test on the cold-rolled steel sheet of each Example, that is, an immersion test with a hydrochloric acid solution was used for the evaluation.
• the wire-cut samples were loaded with stresses of 1.05, 1.1, 1.15, and 1.2 times the tensile strength respectively by bending, and immersed in a 0.1 mol/L HCl solution for 300 h without changing the solution, or a brush was used to remove corrosion products from the surface each time before changing the solution, and the test time was controlled to be 300 h.
• Table 5 lists the test results of the cold-rolled steel sheets of Examples 1-18 after the acid soaking test. Table 5.
• the cold-rolled steel sheets of Examples 1-18 prepared had excellent delayed cracking resistance. Delayed cracking occurred to none of the steel sheets of the Examples when they were immersed in 1 mol/L hydrochloric acid for at least 300 hours under a preset stress of 1.05 times the tensile strength.

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