Cold-rolled ultrahigh-strength dual-phase strip steel with yield strength greater than 650MPa and production method of cold-rolled ultrahigh-strength dual-phase strip steel

Abstract

The invention relates to cold-rolled ultrahigh-strength double-phase strip steel with the yield strength larger than 650 MPa and a production method of the cold-rolled ultrahigh-strength double-phase strip steel. The cold-rolled strip steel comprises the chemical components of 0.07%-0.10% of C, 2.3%-2.5% of Mn, smaller than or equal to 0.005% of S, smaller than or equal to 0.020% of P, 0.50%-0.65% of Si, 0.025%-0.060% of Als, 0.45%-0.60% of Cr, 0.15%-0.25% of Mo, 0.015%-0.030% of Ti and the balance Fe and impurities. According to the longitudinal mechanical properties of the cold-rolled strip steel, the yield strength Rp0.2 is 650-760 MPa, the tensile strength Rm is larger than or equal to 980 MPa, and the percentage elongation after fracture A80 is larger than or equal to 10%. The produced cold-rolled strip steel is good in surface quality, moderate in yield ratio and high in tensile strength, the plastic deformation capacity is equivalent to that of a low-yield-strength product with the same strength level, and the cold-rolled strip steel is particularly suitable for punching automobile safety parts and reinforcing parts and has obvious advantages in the aspect of white automobile body light weight application; meanwhile, production cost is low, production is easy to organize, and economic benefits of enterprises can be improved.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled ultra-high strength steel technology, and in particular to a cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa and its production method. Background Technology

[0002] In recent years, countries worldwide have imposed increasingly stringent requirements on automobiles in terms of environmental protection, safety, and energy conservation and emission reduction. Advanced High Strength Steel (AHSS) has experienced rapid development due to its significant advantages in automotive lightweighting. AHSS is a high-performance steel based on carbon manganese steel, with a tensile strength exceeding 500 MPa. Its development has gone through three generations: the first generation included dual-phase steel (DP), plastic-guided phase transformation steel (TRIP), and multiphase steel (CP); the second generation was represented by twin-induced phase transformation steel (TWIP); and the third generation is represented by quenched and partitioned steel (QP). AHSS utilizes new production processes such as thin-strip continuous casting, overcoming the defects of traditional continuous casting processes that are prone to hot cracking. Currently, due to its high strength and high ductility, AHSS has become an ideal steel for manufacturing automotive components and is widely used in key load-bearing structures such as the A-pillar and B-pillar in safe areas of the vehicle body.

[0003] 980MPa grade cold-rolled high-strength dual-phase steel belongs to the third generation of advanced high-strength steel. After cold rolling, it undergoes continuous annealing and rapid cooling (such as critical annealing + over-aging treatment) to form a dual-phase structure. The microstructure consists of martensite (hard phase) and ferrite (soft phase). By controlling the ratio of the two phases, a combination of high strength and certain plasticity is achieved. It has high strength and lightweight, good formability, collision safety and good fatigue performance. It can significantly improve the safety collision capability of automobiles and the overall rigidity of the vehicle after material thinning. It is increasingly being used in the body of the whole vehicle.

[0004] 980MPa grade cold-rolled high-strength duplex steel can be divided into two main categories based on its lower yield strength: 550MPa and 700MPa. Typically, the higher yield strength of the latter is achieved by increasing the leveling rolling force. This increases the load on the leveling machine and prematurely depletes some of the material's plastic deformation capacity, which is detrimental to subsequent stamping. Conversely, the former, due to its lower yield strength, is detrimental to vehicle body rigidity and collision safety.

[0005] Based on this, through composition and process design, this invention has developed a cold-rolled ultra-high strength duplex strip steel with a yield strength greater than 650MPa. This steel has a moderate yield strength ratio and high tensile strength, while its plastic deformation capacity is comparable to that of products of the same strength grade with low yield strength. This further enriches the product range of 980MPa grade cold-rolled duplex steel, fills a market gap, and its composition system is suitable for production on a continuous annealing line, resulting in low cost and strong market competitiveness. Summary of the Invention

[0006] This invention provides a cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa and its production method. The produced cold-rolled strip steel has good surface quality, a moderate yield strength ratio and high tensile strength, and its plastic deformation capacity is comparable to that of products of the same strength level with low yield strength. It is particularly suitable for stamping automotive safety parts and reinforcements, and has obvious advantages in lightweight applications of body-in-white. At the same time, the production cost is low and production is easy to organize, which is conducive to improving the economic benefits of enterprises.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa. The chemical composition of the cold-rolled strip steel, by mass percentage, is: C: 0.07%–0.10%, Mn: 2.3%–2.5%, S≤0.005%, P≤0.020%, Si: 0.50%–0.65%, Als: 0.025%–0.060%, Cr: 0.45%–0.60%, Mo: 0.15%–0.25%, Ti: 0.015%–0.030%, with the remainder being Fe and unavoidable impurities.

[0009] The microstructure of cold-rolled strip steel consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite. The volume fraction of ferrite is 40% to 50%, the volume fraction of martensite is 45% to 55%, and the remainder is retained austenite.

[0010] The longitudinal mechanical properties of cold-rolled strip steel are: yield strength Rp 0.2 The tensile strength is 650–760 MPa, and the tensile strength R is... m ≥980MPa, elongation after fracture A 80 ≥10%.

[0011] The thickness of cold-rolled strip steel is 1.0 to 2.5 mm, and the width is 1000 to 1450 mm.

[0012] A method for producing cold-rolled ultra-high-strength duplex strip steel with a yield strength greater than 650 MPa includes steelmaking, continuous casting, heating, hot rolling, pickling, continuous annealing, and leveling processes; details are as follows:

[0013] 1) Steelmaking: including hot metal pretreatment, converter smelting, LF refining and RH refining; among which, the hot metal pretreatment inlet temperature is >1200℃; the converter smelting killing time is ≥140s, the converter smelting end temperature is ≥1660℃; the RH refining outlet temperature is 1587~1597℃.

[0014] 2) Continuous casting: The tundish opening temperature is 1529~1549℃, and the casting speed is 1.2~1.5m / min;

[0015] 3) Heating and hot rolling: The heating furnace soaking temperature is 1240-1310℃, and the furnace dwell time is 160-210 min; rough rolling is performed in 3-7 passes, and the thickness of the intermediate slab after rough rolling is 28-40 mm. The exit temperature of the rough rolling mill is 1060-1120℃; after rough rolling, double-manifold descaling is performed with a descaling pressure >20 MPa; the finishing rolling temperature is 860-900℃, and after finishing rolling, double-manifold descaling is performed with a descaling pressure >20 MPa; the coiling temperature is 600-640℃.

[0016] 4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 50% to 68%;

[0017] 5) Continuous annealing: Two-phase zone temperature 770~790℃, slow cooling temperature 630~670℃, rapid cooling zone temperature 270~290℃, over-aging zone temperature 250~270℃, final cooling temperature 150~170℃, belt speed 55~140m / min.

[0018] 6) Leveling: The constant rolling force control mode is adopted, and the rolling force is controlled between 2000 and 3500 kN.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1) The cold-rolled ultra-high strength duplex strip steel of the present invention has a reasonable composition design. The use of Cr+Mo composite can ensure the stability of supercooled austenite and the martensite content after rapid cooling, thereby ensuring that the material reaches the strength level of 980MPa duplex steel.

[0021] 2) This invention adds more low-cost Mn elements and uses only Ti elements for microalloying, which saves production costs while effectively refining grains, strengthening the microstructure, and eliminating rolling microstructure characteristics.

[0022] 3) The cold-rolled strip produced by this invention has good surface quality, moderate yield strength ratio and high tensile strength. Its plastic deformation capacity is comparable to that of products of the same strength level with low yield strength. It is particularly suitable for stamping automotive safety parts and reinforcement parts, and has obvious advantages in lightweight applications of body-in-white. At the same time, the production cost is low and production is easy to organize, which is conducive to improving the economic benefits of enterprises. Attached Figure Description

[0023] Figure 1 This is a microstructure photograph of the cold-rolled ultra-high strength dual-phase strip produced in Example 1 of this invention.

[0024] Figure 2 This is a schematic diagram of the stages of continuous annealing as described in this invention. Detailed Implementation

[0025] The present invention discloses a cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa. The chemical composition of the cold-rolled strip steel, by mass percentage, is: C: 0.07%–0.10%, Mn: 2.3%–2.5%, S≤0.005%, P≤0.020%, Si: 0.50%–0.65%, Als: 0.025%–0.060%, Cr: 0.45%–0.60%, Mo: 0.15%–0.25%, Ti: 0.015%–0.030%, with the remainder being Fe and unavoidable impurities.

[0026] The design principles and functions of key element content in steel are as follows:

[0027] C and Mn are inexpensive austenitizing alloying elements, which mainly play a role in expanding the austenite phase region. They are more likely to generate austenite phase during high-temperature two-phase annealing, ensuring a sufficient proportion of phase transformation strengthening. In addition, considering the weldability of the material, a low-carbon composition design approach is adopted.

[0028] Si: an inexpensive ferritinizing element that is dissolved in ferrite to strengthen the ferrite matrix, mainly playing the role of purifying ferrite and improving yield strength.

[0029] P: Ferritizing element; adding a small amount is beneficial to improving the strength of the ferrite phase.

[0030] Al: Ferritizing element, mainly plays a role in deoxidation, grain refinement and nitrogen atom fixation in the steelmaking process.

[0031] Cr: A stabilizing element in supercooled austenite, which improves the hardenability of materials and is beneficial to improving the tensile strength of materials.

[0032] Mo: A strong element that enhances the stability of austenite, ensuring a sufficient proportion of phase transformation strengthening in the material; at the same time, it can significantly improve the uniformity of the microstructure, ensuring the consistency of the material's mechanical properties.

[0033] Ti: a fine-grained strengthening element, and its precipitated phase precipitates during the hot rolling stage. Its second phase has the effect of dispersed strengthening precipitation, which is beneficial to adjusting the strength and toughness matching of the material.

[0034] The microstructure of cold-rolled strip steel consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite. The volume fraction of ferrite is 40% to 50%, the volume fraction of martensite is 45% to 55%, and the remainder is retained austenite.

[0035] The longitudinal mechanical properties of cold-rolled strip steel are: yield strength Rp 0.2 The tensile strength is 650–760 MPa, and the tensile strength R is... m ≥980MPa, elongation after fracture A 80 ≥10%.

[0036] The thickness of cold-rolled strip steel is 1.0 to 2.5 mm, and the width is 1000 to 1450 mm.

[0037] The present invention discloses a method for producing cold-rolled ultra-high-strength duplex strip steel with a yield strength greater than 650 MPa, comprising steelmaking, continuous casting, heating, hot rolling, pickling, continuous annealing, and leveling processes; specifically as follows:

[0038] 1) Steelmaking: including hot metal pretreatment, converter smelting, LF refining and RH refining; among which, the hot metal pretreatment inlet temperature is >1200℃; the converter smelting killing time is ≥140s, the converter smelting end temperature is ≥1660℃; the RH refining outlet temperature is 1587~1597℃.

[0039] 2) Continuous casting: The tundish opening temperature is 1529~1549℃, and the casting speed is 1.2~1.5m / min;

[0040] 3) Heating and hot rolling: The heating furnace soaking temperature is 1240-1310℃, and the furnace dwell time is 160-210 min; rough rolling is performed in 3-7 passes, and the thickness of the intermediate slab after rough rolling is 28-40 mm. The exit temperature of the rough rolling mill is 1060-1120℃; after rough rolling, double-manifold descaling is performed with a descaling pressure >20 MPa; the finishing rolling temperature is 860-900℃, and after finishing rolling, double-manifold descaling is performed with a descaling pressure >20 MPa; the coiling temperature is 600-640℃.

[0041] 4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 50% to 68%;

[0042] 5) Continuous annealing: Two-phase zone temperature 770~790℃, slow cooling temperature 630~670℃, rapid cooling zone temperature 270~290℃, over-aging zone temperature 250~270℃, final cooling temperature 150~170℃, belt speed 55~140m / min.

[0043] 6) Leveling: The constant rolling force control mode is adopted, and the rolling force is controlled between 2000 and 3500 kN.

[0044] like Figure 2As shown, the cold-rolled steel strip is heated in three stages to the two-phase region for annealing. After a period of time, a certain proportion of austenite and ferrite are generated. Subsequently, in the slow cooling section, a small amount of undercooled austenite transforms into newly formed ferrite, and in the rapid cooling section, a large amount of undercooled austenite transforms into martensite. Then, the martensite is tempered in an aging furnace. Finally, the strip undergoes a leveling process to optimize the plate shape and yield strength. The key to selecting process parameters lies in the selection of the two-phase region annealing temperature and the rapid cooling termination temperature. The former determines the generation of a certain amount of high-temperature austenite, while the latter determines the magnitude of the phase transformation strengthening ratio. Typically, the temperature is selected to be about 60°C below the martensite transformation start temperature. At the same time, a leveling process is used to optimize the plate shape and yield strength, ultimately obtaining a cold-rolled ultra-high-strength dual-phase strip with a good balance of strength and toughness and a yield strength greater than 650MPa.

[0045] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0046]

Example 1

[0047] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 0.9mm (thickness) × 1020mm (width), and its chemical composition and mass percentage content are shown in Table 1.

[0048] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0049] (1) Steelmaking: The inlet temperature of molten iron after pretreatment is 1230℃; the converter smelting calming time is 150s, and the converter smelting endpoint temperature is 1665℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining is 1595℃.

[0050] (2) Continuous casting: The tundish opening temperature is 1535℃ and the casting speed is 1.4m / min.

[0051] (3) Heating and heating: The heating furnace is heated to 1255℃ and the furnace dwell time is 175min; the rough rolling is rolled in 5 passes, the intermediate billet thickness after rough rolling is 30mm, the rough rolling exit temperature is 1070℃, the double-pipe descaling is performed after rough rolling, and the descaling pressure is 25MPa; the finishing rolling temperature is 870℃, the double-pipe descaling is performed after finishing rolling, and the descaling pressure is 23MPa; the coiling temperature is 630℃.

[0052] (4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 60%.

[0053] (5) Continuous annealing: two-phase zone temperature 787℃, slow cooling temperature 650℃, rapid cooling zone temperature 279℃, over-aging zone temperature 262℃, final cooling temperature 159℃, belt speed 132m / min.

[0054] (6) Leveling: The constant rolling force control mode is adopted, and the rolling force is 2000kN.

[0055] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite (e.g., ...). Figure 1 As shown in the figure, the volume fraction of ferrite is 43%, the volume fraction of martensite is 52%, and the remainder is residual austenite.

[0056]

Example 2

[0057] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.0mm (thickness) × 1195mm (width), and its chemical composition and mass percentage content are shown in Table 1.

[0058] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0059] (1) Steelmaking: The inlet temperature of molten iron pretreatment was 1220℃; the converter smelting calming time was 150s, and the converter smelting endpoint temperature was 1670℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining was 1590℃.

[0060] (2) Continuous casting: The tundish opening temperature is 1532℃ and the casting speed is 1.4m / min.

[0061] (3) Heating and hot rolling: The heating furnace is heated to 1250℃ and the furnace dwell time is 170min; the rough rolling is rolled in 4 passes, the intermediate billet thickness after rough rolling is 30mm, the rough rolling exit temperature is 1070℃, the double manifold is descaled after rough rolling, and the descaled pressure is 25MPa; the finishing rolling temperature is 865℃, the double manifold is descaled after finishing rolling, and the descaled pressure is 23MPa; the coiling temperature is 620℃.

[0062] (4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 60%.

[0063] (5) Continuous annealing: two-phase zone temperature 784℃, slow cooling temperature 650℃, rapid cooling zone temperature 280℃, over-aging zone temperature 264℃, final cooling temperature 161℃, belt speed 130m / min.

[0064] (6) Leveling: The constant rolling force control mode is adopted, and the rolling force is 2000kN.

[0065] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0066]

Example 3

[0067] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.2mm (thickness) × 1000mm (width), and its chemical composition and mass percentage content are shown in Table 1.

[0068] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0069] (1) Steelmaking: The temperature of molten iron entering the station after pretreatment is 1230℃; the calming time of converter smelting is 145s, and the final temperature of converter smelting is 1672℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1, and the temperature of RH refining exit is 1592℃.

[0070] (2) Continuous casting: The tundish opening temperature is 1535℃ and the casting speed is 1.4m / min.

[0071] (3) Heating and hot rolling: The heating furnace is heated to 1270℃ and the furnace dwell time is 175min; the rough rolling is rolled in 5 passes, the intermediate billet thickness after rough rolling is 30mm, the rough rolling exit temperature is 1080℃, the double manifold is descaled after rough rolling, and the descaled pressure is 23MPa; the finishing rolling temperature is 880℃, the double manifold is descaled after finishing rolling, and the descaled pressure is 25MPa; the coiling temperature is 630℃.

[0072] (4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 56%.

[0073] (5) Continuous annealing: two-phase zone temperature 785℃, slow cooling temperature 650℃, rapid cooling zone temperature 278℃, over-aging zone temperature 259℃, final cooling temperature 159℃, belt speed 121m / min.

[0074] (6) Leveling: The constant rolling force control mode is adopted, and the rolling force is 2200kN.

[0075] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0076]

Example 4

[0077] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.3mm (thickness) × 1272mm (width), and its chemical composition and mass percentage are shown in Table 1.

[0078] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0079] (1) Steelmaking: The inlet temperature of molten hot metal after pretreatment was 1250℃; the converter smelting calming time was 155s, and the final temperature of the converter smelting was 1675℃; the chemical composition and mass percentage of the molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of the RH refining was 1590℃.

[0080] (2) Continuous casting: The tundish opening temperature is 1536℃ and the casting speed is 1.4m / min.

[0081] (3) Heating and hot rolling: The heating furnace is heated to 1278℃ and the furnace dwell time is 182min; the rough rolling is carried out in 5 passes, the intermediate billet thickness after rough rolling is 30mm, the rough rolling exit temperature is 1085℃, the double-pipe descaling is carried out after rough rolling, and the descaling pressure is 23MPa; the finishing rolling temperature is 885℃, the double-pipe descaling is carried out after finishing rolling, and the descaling pressure is 26MPa; the coiling temperature is 630℃.

[0082] (4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 57%.

[0083] (5) Continuous annealing: two-phase zone temperature 782℃, slow cooling temperature 645℃, rapid cooling zone temperature 284℃, over-aging zone temperature 263℃, final cooling temperature 159℃, belt speed 115m / min.

[0084] (6) Leveling: The constant rolling force control mode is adopted, and the rolling force is 2200kN.

[0085] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0086]

Example 5

[0087] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.4mm (thickness) × 1220mm (width), and its chemical composition and mass percentage content are shown in Table 1.

[0088] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0089] (1) Steelmaking: The inlet temperature of molten iron after pretreatment was 1245℃; the converter smelting calming time was 145s, and the final temperature of converter smelting was 1673℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining was 1592℃.

[0090] (2) Continuous casting: The tundish opening temperature is 1540℃ and the casting speed is 1.4m / min.

[0091] (3) Heating and hot rolling: The heating furnace is heated to 1280℃ and the furnace dwell time is 180min; the rough rolling is rolled in 5 passes, the intermediate billet thickness after rough rolling is 30mm, the rough rolling exit temperature is 1090℃, the double manifold is descaled after rough rolling, and the descaled pressure is 25MPa; the finishing rolling temperature is 875℃, the double manifold is descaled after finishing rolling, and the descaled pressure is 26MPa; the coiling temperature is 615℃.

[0092] (4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 57%.

[0093] (5) Continuous annealing: two-phase zone temperature 788℃, slow cooling temperature 647℃, rapid cooling zone temperature 289℃, over-aging zone temperature 269℃, final cooling temperature 160℃, belt speed 105m / min.

[0094] (6) Leveling: The constant rolling force control mode is adopted, and the rolling force is 2500kN.

[0095] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0096]

Example 6

[0097] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.5mm (thickness) × 1170mm (width), and its chemical composition and mass percentage content are shown in Table 1.

[0098] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0099] (1) Steelmaking: The inlet temperature of molten iron after pretreatment was 1255℃; the converter smelting calming time was 155s, and the final temperature of converter smelting was 1678℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining was 1589℃.

[0100] (2) Continuous casting: The tundish opening temperature is 1535℃ and the casting speed is 1.4m / min.

[0101] (3) Heating and hot rolling: The heating furnace is heated to 1278℃ and the furnace dwell time is 180min; the rough rolling is rolled in 5 passes, the intermediate billet thickness after rough rolling is 30mm, the rough rolling exit temperature is 1088℃, the double manifold is descaled after rough rolling, and the descaled pressure is 26MPa; the finishing rolling temperature is 878℃, the double manifold is descaled after finishing rolling, and the descaled pressure is 24MPa; the coiling temperature is 628℃.

[0102] (4) Pickling and rolling: Cold rolling after pickling, with a cold rolling reduction rate of 53%.

[0103] (5) Continuous annealing: two-phase zone temperature 778℃, slow cooling temperature 646℃, rapid cooling zone temperature 289℃, over-aging zone temperature 265℃, final cooling temperature 161℃, belt speed 102m / min.

[0104] (6) Leveling process: The constant rolling force control mode is adopted, and the rolling force is 2600kN.

[0105] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0106]

Example 7

[0107] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.6mm (thickness) × 1228mm (width), and its chemical composition and mass percentage content are shown in Table 1.

[0108] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0109] (1) Steelmaking: The inlet temperature of molten iron after pretreatment is 1260℃; the converter smelting calming time is 165s, and the converter smelting endpoint temperature is 1682℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining is 1595℃.

[0110] (2) Continuous casting: The tundish opening temperature is 1538℃ and the casting speed is 1.3m / min.

[0111] (3) Heating and hot rolling: The heating furnace is heated to 1288℃ and the furnace dwell time is 185min; the rough rolling is rolled in 5 passes, the intermediate billet thickness after rough rolling is 35mm, the rough rolling exit temperature is 1098℃, the double manifold is used for descaling, and the descaling pressure after rough rolling is 28MPa; the finishing rolling temperature is 888℃, the double manifold is used for descaling after finishing rolling, and the descaling pressure is 26MPa; the coiling temperature is 625℃.

[0112] (4) Pickling and rolling process: after pickling, cold rolling is performed with a cold rolling reduction rate of 54%.

[0113] (5) Continuous annealing: two-phase zone temperature 784℃, slow cooling temperature 648℃, rapid cooling zone temperature 287℃, over-aging zone temperature 261℃, final cooling temperature 158℃, belt speed 95m / min.

[0114] (6) Leveling process: The constant rolling force control mode is adopted, and the rolling force is 2800kN.

[0115] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0116]

Example 8

[0117] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 1.8mm (thickness) × 1220mm (width), and its chemical composition and mass percentage are shown in Table 1.

[0118] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0119] (1) Steelmaking: The inlet temperature of molten iron after pretreatment is 1270℃; the converter smelting and quenching time is 170s, and the final temperature is 1685℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining is 1590℃.

[0120] (2) Continuous casting: The tundish opening temperature is 1535℃ and the casting speed is 1.4m / min.

[0121] (3) Heating and hot rolling: The heating furnace is heated to 1278℃ and the furnace dwell time is 180min; the rough rolling is carried out in 5 passes, the thickness of the intermediate billet after rough rolling is 35mm, the exit temperature of the rough rolling is 1092℃, the double manifold is descaled after rough rolling, and the descaled pressure is 27MPa; the finishing rolling temperature is 885℃, the double manifold is descaled after finishing rolling, and the descaled pressure is 25MPa; the coiling temperature is 635℃.

[0122] (4) Pickling and rolling process: after pickling, the cold rolling reduction rate is 55%.

[0123] (5) Continuous annealing: two-phase zone temperature 781℃, slow cooling temperature 652℃, rapid cooling zone temperature 283℃, over-aging zone temperature 266℃, final cooling temperature 159℃, belt speed 85m / min.

[0124] (6) Leveling process: The constant rolling force control mode is adopted, and the rolling force is 2800kN.

[0125] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0126]

Example 9

[0127] In this embodiment, the finished product specifications of the cold-rolled ultra-high strength dual-phase strip steel are 2.0mm (thickness) × 1258mm (width), and its chemical composition and mass percentage are shown in Table 1.

[0128] In this embodiment, the production process of cold-rolled ultra-high strength dual-phase strip steel includes steelmaking (hot metal pretreatment + converter smelting + LF refining + RH refining), continuous casting, heating, hot rolling, pickling, continuous annealing and leveling processes, and the specific steps are as follows:

[0129] (1) Steelmaking: The inlet temperature of molten iron after pretreatment was 1288℃; the converter smelting calming time was 176s, and the final temperature of converter smelting was 1686℃; the chemical composition and mass percentage of molten steel after LF refining and RH refining are shown in Table 1. The outlet temperature of RH refining was 1593℃.

[0130] (2) Continuous casting: The tundish opening temperature is 1538℃ and the casting speed is 1.3m / min.

[0131] (3) Heating and hot rolling: The heating furnace is heated to 1268℃ and the furnace dwell time is 183min; the rough rolling is carried out in 5 passes, the thickness of the intermediate billet after rough rolling is 35mm, the exit temperature of the rough rolling is 1088℃, the double-pipe descaling is carried out after rough rolling, and the descaling pressure is 26MPa; the finishing rolling temperature is 875℃, the double-pipe descaling is carried out after finishing rolling, and the descaling pressure is 24MPa; the coiling temperature is 632℃.

[0132] (4) Pickling and rolling: After pickling, the cold rolling reduction rate is 50%.

[0133] (5) Continuous annealing: two-phase zone temperature 780℃, slow cooling temperature 656℃, rapid cooling zone temperature 288℃, over-aging zone temperature 264℃, final cooling temperature 161℃, belt speed 75m / min.

[0134] (6) Leveling process: The constant rolling force control mode is adopted, and the rolling force is 3000kN.

[0135] The mechanical properties of the cold-rolled ultra-high strength dual-phase strip steel produced in this embodiment are shown in Table 2. The microstructure consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite.

[0136] Table 1. Composition and weight percentage (wt%) of strip steel in each embodiment.

[0137]

[0138] Table 2 Mechanical properties of finished strip steel products in each embodiment

[0139]

[0140] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa, characterized in that, The chemical composition of cold-rolled strip steel, by mass percentage, is: C: 0.07%–0.10%, Mn: 2.3%–2.5%, S≤0.005%, P≤0.020%, Si: 0.50%–0.65%, Als: 0.025%–0.060%, Cr: 0.45%–0.60%, Mo: 0.15%–0.25%, Ti: 0.015%–0.030%, with the remainder being Fe and unavoidable impurities.

2. The cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa according to claim 1, characterized in that, The microstructure of cold-rolled strip steel consists of a ferrite + martensite dual-phase structure and a small amount of retained austenite. The volume fraction of ferrite is 40% to 50%, the volume fraction of martensite is 45% to 55%, and the remainder is retained austenite.

3. The cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa according to claim 1, characterized in that, The longitudinal mechanical properties of cold-rolled strip steel are: yield strength Rp 0.2 The tensile strength is 650–760 MPa, and the tensile strength R is... m ≥980MPa, elongation after fracture A 80 ≥10%.

4. The cold-rolled ultra-high strength dual-phase strip steel with a yield strength greater than 650 MPa according to claim 1, characterized in that, The thickness of cold-rolled strip steel is 1.0 to 2.5 mm, and the width is 1000 to 1450 mm.

5. A method for producing cold-rolled ultra-high strength duplex strip steel with a yield strength greater than 650 MPa as described in claim 1, 2, 3, or 4, characterized in that, This includes steelmaking, continuous casting, heating, hot rolling, pickling, continuous annealing, and leveling processes; specifically as follows: 1) Steelmaking: including hot metal pretreatment, converter smelting, LF refining and RH refining; among which, the hot metal pretreatment inlet temperature is >1200℃; the converter smelting killing time is ≥140s, the converter smelting end temperature is ≥1660℃; the RH refining outlet temperature is 1587~1597℃. 2) Continuous casting: The tundish opening temperature is 1529~1549℃, and the casting speed is 1.2~1.5m / min; 3) Heating and hot rolling: The heating furnace soaking temperature is 1240-1310℃, and the furnace dwell time is 160-210 min; rough rolling is performed in 3-7 passes, and the thickness of the intermediate slab after rough rolling is 28-40 mm. The exit temperature of the rough rolling mill is 1060-1120℃; after rough rolling, double-manifold descaling is performed with a descaling pressure >20 MPa; the finishing rolling temperature is 860-900℃, and after finishing rolling, double-manifold descaling is performed with a descaling pressure >20 MPa; the coiling temperature is 600-640℃. 4) Pickling and rolling: The cold rolling reduction rate after pickling is 50% to 68%; 5) Continuous annealing: Two-phase zone temperature 770~790℃, slow cooling temperature 630~670℃, rapid cooling zone temperature 270~290℃, over-aging zone temperature 250~270℃, final cooling temperature 150~170℃, belt speed 55~140m / min. 6) Leveling: The constant rolling force control mode is adopted, and the rolling force is controlled between 2000 and 3500 kN.

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