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  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • 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
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  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a metallic material and a method of manufacturing the same, particularly to an electro-galvanized ultra-high-strength dual-phase steel and a method of manufacturing the same.
• Delayed fracture refers to a phenomenon that a material under static stress suffers a sudden brittle failure after a certain period of time. This phenomenon is a kind of embrittlement that occurs when the material interacts with the environmental stress, and it is a form of material deterioration caused by hydrogen.
• the phenomenon of delayed fracture is a major factor that hinders the application of ultra-high-strength steel, and it may be roughly classified into the following two categories:
• the former is generally caused by the intrusion of hydrogen generated by the corrosion reaction at a corrosion pit where corrosion occurs during long-term exposure; while the latter is caused by the concentration of hydrogen in the region of stress concentration under the action of stress wherein the hydrogen intrudes into the steel during manufacturing processes such as pickling and electroplating.
• the slab is heated to 1100 °C or higher and 1300°C or lower, hot-rolled at a finish rolling exit-side temperature of 750 °C or higher and 1000°C or lower, coiled at 300°C or higher and 750°C or lower, and then descaled by pickling.
• the steel plate was held at a temperature ranging from the Ac1 transformation point + 20 °C to the Ac1 transformation point + 120 °C for 600 seconds or more and 21600 seconds or less, and cold rolled at a reduction rate of 30% or more. Then, the steel plate is held at a temperature ranging from the Ac1 transformation point to the Ac1 transformation point + 100 °C for 20 seconds or more and 900 seconds or less, cooled, and then subjected to electro-galvanization.
• the method for producing the cold-rolled steel plate includes the following steps: (1) molten steel pretreatment; (2) converter smelting; (3) alloy fine-tuning station; (4) RH furnace refining; (5) continuous casting; (6) hot rolling; (7) cold rolling; (8) continuous annealing; (9) temper rolling.
• the invention can improve the surface quality of the electro-galvanized steel plate and ensure that the electro-galvanized steel plate has a good shape.
• the mechanical properties of the cold-rolled steel plate are as follows: the yield strength is 120-180 MPa, and the tensile strength is higher than 260 MPa.
• the steel plate is heated to 1250 °C and soaked, and then subjected to three-pass rolling on a finishing mill at a conveying temperature of 900 °C, followed by a heat preservation treatment of 650 °C ⁇ 1 hour.
• the thin steel plate is cold-rolled at a reduction rate of 70 °C/s to obtain a cold-rolled thin steel plate having a thickness of 1.2 mm. This is followed by recrystallization annealing at 850 °C for 60 seconds, cooling at a cooling rate of 30 °C/s, and then electroplating.
• the tensile strength grades of the products involved in the above-mentioned prior art patent documents are all less than 980 MPa, or the matrix is hot stamping steel.
• One of the objects of the present disclosure is to provide an electro-galvanized ultra-high-strength dual-phase steel that is resistant to delayed cracking.
• the composition of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure is designed reasonably. That is, by reasonably designing carbon, silicon, manganese and micro-alloy elements such as niobium, vanadium, chromium, molybdenum and the like in coordination with the process, the resulting steel has both excellent resistance to delayed cracking and ultra-high strength.
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking has a yield strength of ⁇ 550 MPa, a tensile strength of ⁇ 980 MPa, an elongation after fracture of ⁇ 12%, an initial hydrogen content of ⁇ 3 ppm, preferably ⁇ 2 ppm, and it does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of 1.2 times the tensile strength.
• the excellent performances of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking can meet the industrial requirements, and be used for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.
• the present disclosure provides an electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking, having a matrix structure of ferrite+tempered martensite, wherein the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking comprises the following chemical elements in mass percentages, in addition to Fe: C: 0.07-0.1%, Si: 0.05-0.3%, Mn: 2.0-2.6%, Cr: 0.2-0.6%, Mo: 0.1-0.25%, Al: 0.02-0.05%, Nb: 0.02-0.04%, V: 0.06-0.2%.
• the mass percentages of the chemical elements are: C: 0.07-0.1%, Si: 0.05-0.3%, Mn: 2.0-2.6%, Cr: 0.2-0.6%, Mo: 0.1-0.25%, Al: 0.02-0.05%, Nb: 0.02-0.04%, V: 0.06-0.2%, and a balance of Fe and other unavoidable impurities.
• the chemical elements are designed according to the following principles:
• electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure further comprises 0.0015-0.003% of element B.
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure may also comprise a small amount of element B.
• B is used as a strong element for hardenability.
• An appropriate amount of B can increase the hardenability of the steel, and promote formation of martensite.
• 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 ⁇ 0.012%, S ⁇ 0.003%, N ⁇ 0.005%.
• P, S and N are all unavoidable impurity elements in the steel. It's better to lower the contents of the P, S and N elements in the steel as far as possible. S tends to form MnS inclusions which will seriously affect the hole expansion rate.
• the P element may reduce the toughness of the steel, which is not conducive to the delayed cracking performance. An unduly high content of the N element in the steel is prone to causing cracks on the surface of the slab, which will greatly affect the performances of the steel.
• the mass percentage of P is controlled at P ⁇ 0.012%; 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 the tempered martensite is >50%.
• the carbide particles include MoC, VC, Nb (C, N), and the carbide particles are all distributed in the matrix structure in a coherent form.
• the carbide particles have a size of ⁇ 60 nm.
• the tempered martensite further comprises coherently distributed ⁇ carbides.
• the performances of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking meet at least one of the following: yield strength ⁇ 550 MPa, tensile strength ⁇ 980 MPa, elongation after fracture ⁇ 12%, initial hydrogen content ⁇ 3 ppm; no delayed cracking when soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
• the performances of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking meet the following: yield strength ⁇ 550 MPa, tensile strength ⁇ 980 MPa, elongation after fracture ⁇ 12%, initial hydrogen content ⁇ 3 ppm; no delayed cracking when soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
• the yield ratio of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure is in the range of 0.55-0.70.
• another object of the present disclosure is to provide a method for manufacturing an electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking.
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking manufactured by this method has a yield strength of ⁇ 550 MPa, a tensile strength of ⁇ 980MPa, an elongation after fracture of ⁇ 12%, an initial hydrogen content of ⁇ 3ppm, preferably ⁇ 2ppm, and it does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
• the present disclosure provides a method for manufacturing the above electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking, comprising steps:
• a medium to low temperature tempering treatment is utilized to control the relevant process parameters. This not only helps to reduce the hardness of martensite, but can also effectively avoid precipitation of coarse-grained martensite, which is very beneficial to the delayed cracking performance of the steel.
• step (1) a drawing speed in the continuous casting is controlled at 0.9-1.5 m/min.
• step (1) the continuous casting may be performed in a secondary cooling mode with a large amount of water.
• step (2) the cast slab is controlled to be soaked at a temperature of 1200-1260 °C, preferably 1210-1245 °C; then rolled with a finishing rolling temperature being controlled at 840-900 °C; then cooled at a rate of 20-70 °C/s after rolling; then coiled at a coiling temperature of 580-630 °C; and then subjected to heat preservation treatment or slow cooling treatment after coiling.
• the heat preservation treatment is performed for 1-5 hours, or the slow cooling treatment is performed at a cooling rate of 3-5 °C/s.
• step (2) in order to ensure the stability of the rolling load, the heating temperature is controlled at 1200 °C or higher. Meanwhile, the upper limit of the heating temperature is controlled to be 1260 °C in order to prevent increase of oxidative burning loss. Therefore, the cast slab is finally controlled to be soaked at a temperature of 1200-1260 °C.
• step (2) the heat preservation after hot-rolling and coiling or the slow cooling after coiling is conducive to full precipitation of dispersive precipitates.
• Various types of dispersively distributed precipitates are conducive to adsorption of a small amount of hydrogen and dispersive distribution of hydrogen, thereby avoiding gathering of hydrogen. This helps to resist delayed cracking.
• step (3) the cold rolling reduction rate is controlled at 45-65%.
• step (3) the cold rolling reduction rate is controlled at 45-65%. Before the cold rolling, the iron oxide scale on the surface of the steel plate can be removed by pickling.
• step (6) the temper rolling reduction rate is controlled at ⁇ 0.3%.
• step (6) in order to guarantee the flatness of the steel plate, a certain amount of temper rolling needs to be performed, but an excessively large amount of temper rolling will increase the yield strength of the steel too much. Therefore, in the manufacturing method according to the present disclosure, the temper rolling reduction rate is controlled at ⁇ 0.3%.
• step (7) may be performed by a conventional electro-galvanizing method.
• double-side plating is performed, and the weight of the plating layer on one side is in the range of 10-100 g/m 2 .
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking and the method of manufacturing the same according to the present disclosure have the following advantages and beneficial effects:
• the composition of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure is designed reasonably. That is, by reasonably designing carbon, silicon, manganese and micro-alloy elements such as niobium, vanadium, chromium, molybdenum and the like in coordination with the process, the resulting steel has both excellent resistance to delayed cracking and ultra-high strength.
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking has a yield strength of ⁇ 550 MPa, a tensile strength of ⁇ 980 MPa, an elongation after fracture of ⁇ 12%, an initial hydrogen content of ⁇ 3 ppm, and delayed cracking does not occur when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
• the excellent performances of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking can meet the industrial requirements, suitable for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.
• the interior of the steel plate, especially the surface layer is free of TiN, which is conducive to reducing gathering of hydrogen in the interior of the steel plate and improving the delayed cracking performance of the steel.
• a combination of high temperature soaking and medium temperature tempering is adopted.
• the high temperature soaking gives rise to more austenite transformation, and thus more martensite is obtained during the subsequent rapid cooling, which finally guarantees higher strength before tempering.
• medium to low temperature tempering treatment and controlling relevant process parameters not only reduction of the hardness of martensite is favored, but precipitation of coarse-grained martensite is also avoided effectively, so that the yield ratio of the material is moderate, and on the other hand, the delayed cracking performance of the steel is greatly favored.
• the tempering temperature used is too low, it is not conducive to reducing the hardness of martensite; if the tempering temperature is too high, martensite will decompose, and the final strength will be lower than 980 MPa.
• the use of high temperature soaking and medium temperature tempering in combination according to the present disclosure effectively ensures that the prepared electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking has the characteristics of excellent delayed-cracking resistance and low initial hydrogen content.
• Table 1 lists the mass percentages of various chemical elements in the steel grades corresponding to the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14. Table 1 wt%, the balance is Fe and other unavoidable impurities except for P, S and N) Steel grade C Si Mn P S Nb Cr Mo Al N V B Ex. 1 A 0.07 0.05 2.07 0.011 0.001 0.022 0.34 0.21 0.021 0.0035 0.11 0.0015 Ex. 2 B 0.073 0.08 2.14 0.008 0.0008 0.024 0.38 0.23 0.033 0.0044 0.15 0.0020 Ex.
• Tables 2-1 and 2-2 list the specific process parameters for the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14.
• Step (4) Step (5) Step (6) Heating rate (°C/s) Annealing soaking temperature (°C) Annealing time (s) Rapid cooling rate (°C/s) Starting temperature of rapid cooling (°C) Tempering temperature (°C) Tempering time (s) Temper rolling reduction rate (%) Ex. 1 5 795 60 55 710 260 100 0.1 Ex. 2 8 790 80 35 680 240 300 0.1 Ex. 3 7 785 120 80 650 210 250 0.3 Ex. 4 4 794 160 45 730 270 200 0.1 Ex.
• Table 3 lists the performance test results for the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14.
• GB/T 13239-2006 Metallic Materials - Tensile Testing At Low Temperature was referred to.
• a standard sample was prepared, and subjected to static stretching on a tensile testing machine to obtain a corresponding stress-strain curve. After data processing, the parameters of yield strength, tensile strength and elongation after fracture were obtained finally.
• each Example according to the present disclosure had a yield strength of ⁇ 550 MPa, a tensile strength of ⁇ 980 MPa, an elongation after fracture of ⁇ 12%, and an initial hydrogen content of ⁇ 3 ppm.
• the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking in each Example had an ultra-high strength and a delayed cracking performance that was significantly better than that of a comparative steel grade of the same level. No delayed cracking occurred when the steel plate was soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
• the excellent performances of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure can meet the industrial requirements, suitable for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.

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