Production method of low-cost cold-rolled dual-phase steel DP980 with yield strength of 550-760 MPa

Abstract

The invention discloses a production method of low-cost cold-rolled dual-phase steel DP980 with yield strength of 550-760 MPa, which comprises the following steps: (1) smelting and continuous casting: molten steel for a casting machine comprises the following components in percentage by mass: 0.090-0.120% of C, 0.40-0.60% of Si, 2.40-3.00% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S, 0.015-0.045% of Alt, 0.10-0.30% of Cr, 0.020-0.045% of Nb, 0.015-0.035% of Ti, 0.0008-0.0030% of Ca and 0.0030-0.0070% of N; (2) hot rolling; (3) pickling and cold rolling; and (4) continuous annealing and leveling. The cold-rolled dual-phase steel disclosed by the invention has the characteristics of low yield ratio, high elongation percentage, excellent formability and the like.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical sheet technology, and particularly relates to a low-cost production method for cold-rolled duplex steel DP980 with a yield strength of 550~760MPa. Background Technology

[0002] With the rapid development of the automotive industry towards lightweighting and enhanced safety, higher requirements are being placed on the strength, plasticity, and formability of steel used in car bodies. DP980, a cold-rolled duplex steel with a yield strength of 550–760 MPa, possesses an excellent balance of high strength and good cold formability, effectively reducing vehicle weight and improving collision safety. It has become a core material for mid-to-high-end automotive body structural components and safety components (such as anti-collision beams and door anti-collision bars).

[0003] The current production of DP980 generally faces the industry pain point of "difficulty in balancing high performance and low cost." To achieve the target strength, traditional processes often rely on adding precious alloying elements such as Mo and V to improve hardenability, leading to a significant increase in material costs. Furthermore, high alloy content exacerbates rolling deformation resistance, causing defects such as strip breakage and edge cracking, thus reducing yield. Data shows that the addition of precious alloys accounts for more than 15% of the material cost of DP980, while the price of imported high-performance DP980 is about 20% higher than that of domestically produced ordinary high-strength steel, severely restricting its large-scale application.

[0004] At the process level, cost-cutting modifications can easily lead to performance fluctuations and production stability issues. Simply reducing alloy content without corresponding process optimization can result in insufficient or uneven martensite content, causing the yield strength to deviate from the target range of 550–760 MPa, and reducing the strength-ductility balance. During continuous annealing, issues such as reduced furnace roll roughness and tension fluctuations can easily cause strip misalignment and surface damage, while the pickling difficulties caused by high silicon content can lead to under-pickling, black spots, and other surface defects, further reducing the product yield. In addition, traditional two-step fractionation methods and other processes have high equipment requirements, resulting in high equipment investment and energy costs, making it difficult to meet the needs of low-cost industrial production.

[0005] While existing technologies have attempted to reduce costs through composition optimization, such as using Mo-free designs or Nb-Ti composite microalloying to replace precious elements, these methods primarily target single production stages and lack a coordinated control scheme for parameters across the entire process of smelting, hot rolling, cold rolling, and annealing. Although some solutions have reduced alloy costs, they have led to increased rolling difficulty or greater performance fluctuations (strength fluctuations exceeding 10%), failing to meet the stringent performance consistency requirements of downstream high-end manufacturing. Therefore, developing a low-cost DP980 production method that can reduce reliance on precious alloys through precise composition design while ensuring stable yield strength and production efficiency through coordinated process control has become crucial for steel companies to enhance their market competitiveness and adapt to the demands of high-end manufacturing.

[0006] Currently, the 980MPa grade cold-rolled duplex steel sheet produced by Maanshan Iron & Steel (Group) Holding Co., Ltd. (a 980MPa grade cold-rolled duplex steel sheet with excellent cold bending performance and its preparation method, application publication number CN 107043888 A) has a Nb content of 0.050-0.075% and a Ti content of 0.060-0.10% in its composition design. The higher Nb and Ti content increases the product cost, which differs from the composition of this invention; Chinese patent CN 107058869, published on May 31, 2019. B discloses an ultra-low yield strength ratio 980MPa grade cold-rolled dual-phase steel and its manufacturing method. In the steel plate composition design, C: 0.13~0.18%, the high C content is not conducive to welding and rolling, which differs from the composition of this invention. The 980MPa grade low carbon cold-rolled dual-phase steel produced by Panzhihua Iron and Steel Research Institute Co., Ltd. of Panzhihua Iron and Steel Group (980MPa grade low carbon cold-rolled dual-phase steel and its preparation method, application publication number CN109280854A) has a steel plate composition design that, although the method reduces the C content, adds a large amount of alloying elements V and Cr, V: 0.05~0.15%, Cr: 0.40~0.80%, which affects welding performance and increases alloy cost, which also differs from the composition of this invention. Summary of the Invention

[0007] The purpose of this invention is to provide a low-cost production method for cold-rolled duplex steel DP980 with a yield strength of 550~760MPa. This cold-rolled duplex steel possesses characteristics such as a low yield strength ratio, high elongation, and excellent formability, meeting users' high-end material selection needs and enabling applications such as lightweight and high-strength vehicle body structures. Its mechanical properties meet the following requirements: yield strength R... p0.2 550~760MPa, tensile strength R m ≥980MPa, elongation A 80mm ≥10%.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0009] This invention discloses a low-cost production method for cold-rolled duplex steel DP980 with a yield strength of 550~760MPa, comprising:

[0010] (1) Smelting-continuous casting production process: hot metal pretreatment—converter—LF refining—RH vacuum treatment—slab continuous casting; oxygen blowing and decarburization are carried out in the top and bottom blowing smelting of the converter, and the tapping temperature is ≥1620℃ to ensure that the composition and temperature are coordinated when tapping; if the tapping temperature of the new tapping outlet is increased by 10~15℃ on the basis of the upper limit of temperature; aluminum ferromanganese ferrosilicon ferrochrome alloy is added during the tapping process of the converter for deoxidation and alloying, and the P and S composition is controlled to prevent the steel liquid from over-oxidation; LF furnace construction Slag desulfurization is performed, and aluminum ferrosilicon, ferromanganese, ferrochrome, and ferroniobium alloys are added to adjust the composition to the target range; precise carbon control is implemented in the RH furnace, stabilizing C to the 0.090~0.120% required for DP980, avoiding C content fluctuations that affect the martensite to ferrite ratio; deep degassing ensures a pure degassing time of ≥7min, and R dehydrogenation to ≤4ppm to reduce hydrogen-induced cracking; temperature is fine-tuned to provide stable superheat for continuous casting, connecting the converter-LF-continuous casting process to ensure smooth production; vacuum degree requirement ≤ 2.6 mbar, maintain vacuum time ≥10 min; during RH vacuum treatment, add ferro-titanium alloys to adjust the composition according to the steel composition; after RH vacuum treatment, feed in calcium wire for calcium treatment, and ensure soft blowing time ≥9 min after wire feeding to promote inclusion flotation and ensure steel purity; adjust to the target composition according to the steel composition, the mass percentage composition of the steel supplied to the casting machine is C: 0.090~0.120%, Si: 0.40~0.60%, Mn: 2.40~ 3.00%, P≤0.015%, S≤0.015%, Alt: 0.015~0.045%, Cr: 0.10~0.30%, Nb: 0.020~0.045%, Ti: 0.015~0.035%, Ca: 0.0008~0.0030%, N: 0.0030~0.0070%, with the remainder being Fe and impurities; slab continuous casting superheated to 15~40℃, casting speed controlled at 1.0~1.8m / min;

[0011] (2) Hot rolling production process: billet heating—high-pressure water descaling—width-fixed press—rough rolling—flying shearing of head and tail—high-pressure water descaling—finish rolling—laminar cooling—coiling; billet time in furnace is 180~

[0012] The hot-rolled strip is rolled over a 240-minute period at a furnace temperature of 1220-1280℃. Roughing is performed on a 3+3 mode 2-stand mill, and finishing is done on a 7-stand mill. The initial finishing temperature is 960-1050℃, and the final finishing temperature is 860-920℃. The hot-rolled strip thickness is 2.10-4.80 mm. Cooling is achieved using laminar flow cooling equipment, with a coiling temperature of 550-620℃.

[0013] (3) Pickling and cold rolling process: pickling and uncoiling → welding → tension leveling → pickling → rinsing → drying → trimming → rolling → coiling → weighing → marking → bundling → warehousing. The process section adopts shallow trough turbulent pickling process; after the hot-rolled steel strip is pickled to remove the surface iron oxide scale, it is rolled to the order thickness by a 5-stand rolling mill; the acid temperature is 70~90℃, the concentration of No. 1 free acid is 40~90g / L, and the concentration of No. 2 free acid is 70~100g / L. The concentration of free acid #3 is 95-150 g / L; the rinsing water temperature is 60-80℃; the drying temperature is 100-130℃; and the pickling speed is ≥30 m / min. The total cold rolling reduction is 47-60%, the rolling thickness is 0.90-2.00 mm, and it is completed in 4 passes. The reduction rate is 25-45% in the first and second passes, 15-25% in the third pass, and 8-12% in the last pass. The coiling tension is 43-50 N / mm. 2 ;

[0014] (4) Continuous annealing and leveling process: continuous annealing uncoiling → welding → cleaning → annealing furnace → leveling → edge trimming → surface inspection → oiling → sampling and inspection → coiling → weighing → marking → bundling → packaging → warehousing. After uncoiling the cold-hardened steel strip, welding, cleaning, annealing and leveling are carried out continuously. The alkaline solution temperature is controlled at 55~85℃, the conductivity of 1# and 2# alkaline washing is 20~45ms, the conductivity of electrolytic cleaning is 45~85ms, the drying temperature is 100~135℃, the furnace speed is 60~160m / min, the soaking temperature is 780~820℃, slow cooling to 650~680℃, then cooling to 280~310℃, the over-aging temperature is 270~300℃, and the leveling elongation is 0.20%~0.50%.

[0015] Furthermore, the cold-rolled duplex steel DP980 has the following composition by mass percentage: C: 0.090%, Si: 0.52%, Mn: 2.70%, P: 0.015%, S: 0.002%, Alt: 0.036%, Cr: 0.23%, Nb: 0.020%, Ti: 0.015%, Ca: 0.0030%, N: 0.0036%, with the remainder being Fe and impurities.

[0016] Furthermore, the cold-rolled duplex steel DP980 has the following composition by mass percentage: C: 0.095%, Si: 0.50%, Mn: 2.62%, P: 0.010%, S: 0.005%, Alt: 0.030%, Cr: 0.20%, Nb: 0.030%, Ti: 0.020%, Ca: 0.0020%, N: 0.0035%, with the remainder being Fe and impurities. It does not contain expensive alloying elements such as Mo and V; the strengthening effect of expensive alloys is replaced by the synergistic effect of Mn and Cr, thereby reducing raw material costs.

[0017] Furthermore, the hot-rolled thickness is 2.30~3.80mm.

[0018] Furthermore, the furnace exit temperature is 1250~1270℃.

[0019] Furthermore, the initial rolling temperature for finishing rolling is 960-1050℃, and the final rolling temperature for finishing rolling is 875-900℃.

[0020] Furthermore, the winding temperature is 560-590℃.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0022] This invention relates to 980MPa grade cold-rolled duplex steel, which possesses characteristics such as low yield strength ratio, high elongation, and excellent formability. It can meet users' high-end material selection needs and enable applications such as lightweight and high-strength vehicle body structures. Mechanical properties: Yield strength R... p0.2 550~760MPa, tensile strength R m ≥980MPa, elongation A 80mm ≥10%. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings.

[0024] Figure 1 Heating curve for cold-rolled duplex steel;

[0025] Figure 2 This is a microstructure diagram of an embodiment of the present invention. Detailed Implementation

[0026] The present invention will be described in detail below through specific embodiments. These embodiments are only intended to help understand the present invention and do not limit the scope of the present invention.

[0027] Example

[0028] According to the above steelmaking process requirements, the actual chemical composition of the slab is shown in Table 1.

[0029] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0030]

[0031] According to the above hot rolling process requirements, the hot rolling thickness is 2.30~3.80mm, and the actual process is shown in Table 2.

[0032] Table 2 Hot rolling process of embodiments of the present invention

[0033]

[0034] According to the above requirements for cold rolling annealing and leveling processes, the cold rolling thickness is 0.90~2.00mm. The actual cold rolling annealing and leveling processes are shown in Table 3.

[0035] Continuous annealing and leveling process: After uncoiling the chilled hardened steel strip, continuous welding is carried out. The furnace speed is 60~160m / min, the soaking temperature is 780~820℃, the slow cooling is 650~680℃, and then the cooling is 280~310℃. The over-aging temperature is 270~300℃, and the leveling elongation is 0.20%~0.40%.

[0036] Table 3 Annealing and leveling processes in embodiments of the present invention

[0037]

[0038] The mechanical properties of the steel strip in this embodiment of the invention were tested, and the test results are shown in Table 4.

[0039] Table 4 Mechanical properties of steel strips in embodiments of the present invention

[0040]

[0041] As shown in Table 4, the mechanical properties of the low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to the present invention are as follows: Example 1: Yield strength R p0.2 660~745MPa, tensile strength R m 1031~1148MPa, elongation A 80mm : 10.07~14.46%; Example 2: Yield strength R p0.2 648~696MPa, tensile strength R m 1020~1118MPa, elongation A 80mm The mechanical properties of the steel strip products obtained by the processes of Example 1 and Example 2 are 10.64~15.45%. In summary, the mechanical properties of the steel strip products obtained by the processes of Example 1 and Example 2 meet the requirements. After being used by users, the performance is suitable and can be promoted for use.

[0042] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A low-cost production method for cold-rolled duplex steel DP980 with a yield strength of 550~760MPa, characterized in that: include: (1) Smelting-continuous casting production process: hot metal pretreatment—converter—LF refining—RH vacuum treatment—slab continuous casting; In the converter top and bottom blowing smelting process, oxygen blowing is used for decarburization and heating, with a tapping temperature ≥1620℃ to ensure that the composition and temperature are coordinated during tapping. If the tapping temperature at the new tapping port is higher than the upper limit, it is increased by 10~15℃ as appropriate. During the converter tapping process, aluminum ferromanganese, ferrosilicon, and ferrochrome alloys are added for deoxidation and alloying, controlling the P and S composition to prevent over-oxidation of the molten steel. In the LF furnace, slag is formed and desulfurized, and aluminum ferromanganese, ferrosilicon, ferrochrome, and ferroniobium alloys are added to adjust to the target composition range. In the RH furnace, carbon is precisely controlled to stabilize C to 0.090~0.120% required for DP980, avoiding the impact of C content fluctuations on the martensite and ferrite ratio. Deep degassing is carried out to ensure that the pure degassing time is ≥7min, and R dehydrogenation is reduced to ≤4ppm to reduce hydrogen-induced cracking. The temperature is finely adjusted to provide stable superheat for continuous casting, connecting the converter-LF-continuous casting process to ensure smooth production. The vacuum degree requirement is ≤2.6mbar, and the vacuum time is maintained for ≥10min. During RH vacuum treatment, alloys such as ferrotitanium are added to adjust the composition of the molten steel. After RH vacuum treatment, calcium wire is fed in for calcium treatment. After feeding the wire, the soft blowing time is guaranteed to be ≥9 minutes to promote the flotation of inclusions and ensure the purity of the molten steel. The composition is adjusted to the target composition according to the molten steel composition. The mass percentage composition of the molten steel supplied to the casting machine is C: 0.090~0.120%, Si: 0.40~0.60%, Mn: 2.40~3.00%, P≤0. 0.015%, S≤0.015%, Alt: 0.015~0.045%, Cr: 0.10~0.30%, Nb: 0.020~0.045%, Ti: 0.015~0.035%, Ca: 0.0008~0.0030%, N: 0.0030~0.0070%, with the remainder being Fe and impurities; slab continuous casting superheated to 15~40℃, casting speed controlled at 1.0~1.8m / min; (2) Hot rolling production process: billet heating—high-pressure water descaling—width-fixed press—rough rolling—flying shearing of head and tail—high-pressure water descaling—finish rolling—laminar cooling—coiling; billet time in furnace is 180~ The hot-rolled strip is rolled over a 240-minute period at a furnace temperature of 1220-1280℃. Roughing is performed on a 3+3 mode 2-stand mill, and finishing is done on a 7-stand mill. The initial finishing temperature is 960-1050℃, and the final finishing temperature is 860-920℃. The hot-rolled strip thickness is 2.10-4.80 mm. Cooling is achieved using laminar flow cooling equipment, with a coiling temperature of 550-620℃. (3) Pickling and cold rolling process: pickling and uncoiling → welding → tension leveling → pickling → rinsing → drying → trimming → rolling → coiling → weighing → marking → bundling → warehousing. The process section adopts shallow trough turbulent pickling process; after the hot-rolled steel strip is pickled to remove the surface iron oxide scale, it is rolled to the order thickness by a 5-stand rolling mill; the acid temperature is 70~90℃, the concentration of No. 1 free acid is 40~90g / L, and the concentration of No. 2 free acid is 70~100g / L. The concentration of free acid #3 is 95-150 g / L; the rinsing water temperature is 60-80℃; the drying temperature is 100-130℃; and the pickling speed is ≥30 m / min. The total cold rolling reduction is 47-60%, the rolling thickness is 0.90-2.00 mm, and it is completed in 4 passes. The reduction rate is 25-45% in the first and second passes, 15-25% in the third pass, and 8-12% in the last pass. The coiling tension is 43-50 N / mm. 2 ; (4) Continuous annealing and leveling process: continuous annealing uncoiling → welding → cleaning → annealing furnace → leveling → edge trimming → surface inspection → oiling → sampling and inspection → coiling → weighing → marking → bundling → packaging → warehousing. After uncoiling the cold-hardened steel strip, welding, cleaning, annealing and leveling are carried out continuously. The alkaline solution temperature is controlled at 55~85℃, the conductivity of 1# and 2# alkaline washing is 20~45ms, the conductivity of electrolytic cleaning is 45~85ms, the drying temperature is 100~135℃, the furnace speed is 60~160m / min, the soaking temperature is 780~820℃, slow cooling to 650~680℃, then cooling to 280~310℃, the over-aging temperature is 270~300℃, and the leveling elongation is 0.20%~0.50%.

2. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: The cold-rolled duplex steel DP980 has the following composition by mass percentage: C: 0.090%, Si: 0.52%, Mn: 2.70%, P: 0.015%, S: 0.002%, Alt: 0.036%, Cr: 0.23%, Nb: 0.020%, Ti: 0.015%, Ca: 0.0030%, N: 0.0036%, with the remainder being Fe and impurities.

3. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: The cold-rolled duplex steel DP980 has the following composition by mass percentage: C: 0.095%, Si: 0.50%, Mn: 2.62%, P: 0.010%, S: 0.005%, Alt: 0.030%, Cr: 0.20%, Nb: 0.030%, Ti: 0.020%, Ca: 0.0020%, N: 0.0035%, with the remainder being Fe and impurities.

4. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: Hot-rolled thickness 2.30~3.80mm.

5. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: The furnace temperature is 1250~1270℃.

6. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: The initial rolling temperature for finishing rolling is 960-1050℃, and the final rolling temperature for finishing rolling is 875-900℃.

7. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: Winding temperature 560-590℃.

8. The method for producing low-cost cold-rolled duplex steel DP980 with a yield strength of 550~760MPa according to claim 1, characterized in that: Mechanical properties satisfy: Yield strength R p0.2 550~760MPa, tensile strength R m ≥980MPa, elongation A 80mm ≥10%.

Images