980mpa grade reinforced forming cold-rolled multiphase steel and a method for manufacturing the same

Abstract

The application discloses a 980MPa reinforced forming cold-rolled multiphase steel and a preparation method thereof. The chemical components of the multiphase steel are as follows in percentage by mass: C: 0.14-0.16%, Si: 0.6-0.8%, Mn: 2.2-2.5%, P: less than or equal to 0.015%, S: less than or equal to 0.005%, Cr: 0.28-0.32%, Nb: 0.04-0.05%, Ti: 0.03-0.04%, Mo: 0.18-0.23%, V: 0.01-0.02%, and the rest is Fe and inevitable impurities. The preparation method comprises smelting, continuous casting, hot rolling, acid rolling, continuous annealing and finishing processes. The multiphase steel provided by the application has a tensile strength of more than or equal to 980MPa, a yield strength of more than or equal to 780MPa, an elongation A50 of more than or equal to 11%, a hole expansion ratio of more than or equal to 45%, and no cracks in 0T cold bending in the transverse direction.

Description

Technical Field

[0001] This invention belongs to the field of cold-rolled sheet and strip production technology, specifically relating to a 980MPa grade reinforced cold-rolled multiphase steel and its preparation method. Background Technology

[0002] As the automotive industry upgrades towards lightweighting and high safety, 980MPa-grade reinforced multiphase steel (CH980) has become a core material for vehicle bodies due to its combination of high strength and good formability. However, existing technologies face three major bottlenecks: First, the formability is uneven, with some products failing to achieve a hole expansion rate of 45%, and there are no clear requirements for bending performance; especially in the forming process of complex automotive parts, the bending crack rate is as high as 30% or more, which cannot meet the current needs of automotive parts; second, the microstructure control is coarse, with the strength difference between ferrite and hard phase (bainite / martensite) exceeding 500MPa, leading to stress concentration and cracking during local forming; third, the production process has poor stability, requiring special processes such as secondary cold rolling and quenching and partitioning, which significantly increases the production cost and makes large-scale mass production difficult.

[0003] Existing technology CN 116987859A employs a process of "hot rolling → single cold rolling → bell-type annealing → double cold rolling → continuous annealing," which is complex and does not involve bending performance evaluation. While CN 115572899A discloses an annealing method for cold-rolled multiphase steel, its tensile strength is only 880 MPa, and it does not solve the problem of bending brittleness. Although CN 112251668A discloses the composition of reinforced multiphase steel, it does not introduce toughness-regulating elements, and the process does not achieve control over the gradient distribution of retained austenite, thus failing to achieve good bending stability.

[0004] Therefore, the development of CH980 steel, which combines ultra-high hole expansion toughness, good bending stability, and broad process applicability, has become an urgent need in the industry. Summary of the Invention

[0005] To address the problems of insufficient strength, lack of bending performance, easy cracking during forming, and complex production process of existing reinforced and formed multiphase steel, this invention aims to provide a 980MPa grade reinforced and formed cold-rolled multiphase steel and its preparation method, which can achieve ultra-high hole expansion ratio (≥45%), crack-free transverse 0T bending, and can be stably produced on conventional pickling and rolling continuous annealing production lines.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A 980MPa grade reinforced cold-rolled multiphase steel has the following chemical composition by mass percentage: C: 0.14-0.16%, Si: 0.6-0.8%, Mn: 2.2-2.5%, P≤0.015%, S≤0.005%, Cr: 0.28-0.32%, Nb: 0.04-0.05%, Ti: 0.03-0.04%, Mo: 0.18-0.23%, V: 0.01-0.02%, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, the microstructure of the 980MPa grade reinforced cold-rolled multiphase steel consists of 55-65% bainite, 20-25% ferrite, 10-15% martensite and 3-5% retained austenite, with an average grain size ≤5μm, and the volume fraction of retained austenite at the grain boundaries is more than 20% higher than that within the grains.

[0009] The preparation method of the 980MPa grade reinforced cold-rolled multiphase steel of the present invention includes the following steps:

[0010] 1) Smelting: Through converter smelting + LF refining + RH refining, the target composition of molten steel is obtained by controlling [H] ≤ 2ppm and [O] ≤ 15ppm.

[0011] 2) Continuous casting: Dynamic light reduction technology is adopted, the casting temperature is 1505-1515℃, the superheat of molten steel is 15-20℃, the thickness of the continuous casting billet is 210-240mm, and the slow cooling time of the billet is ≥48h.

[0012] 3) Hot rolling: Cold billets are loaded into the furnace and heated to 1200-1250℃ for 200-220 minutes to ensure full solution treatment of various alloys; the initial rolling temperature of finishing rolling is 1050-1090℃, and the final rolling temperature is 910-930℃; laminar cooling adopts a front-stage rapid cooling + rear-stage slow cooling mode, with a coiling temperature of 590-610℃ and a 50℃ temperature compensation in the 100mm area at the beginning and end of the strip to ensure the uniformity of the longitudinal microstructure of the hot-rolled coil; the thickness of the hot-rolled coil is 2.6-3.3mm.

[0013] 4) Pickling and rolling: After the hot-rolled coil is pickled to remove the surface iron oxide scale, it is cold-rolled with a reduction rate of 53-56% to obtain a cold-rolled coil with a thickness of 1.2-1.4mm.

[0014] 5) Continuous annealing: The process of "gradient heating - precise heat homogenization - segmented controlled cooling" is adopted: heating to a homogenization temperature of 850-865℃ and holding for 160-180s; slow cooling to 695-705℃ with a cooling rate of ≤5℃ / s; rapid cooling to 360-380℃ with a cooling rate of 28-32℃ / s; over-aging temperature of 310-330℃ and holding for 360-420s; and final cooling to below 80℃ for leveling with a leveling elongation of 0.25-0.35%.

[0015] The inventive principle of the technical solution of this invention lies in:

[0016] (1) Precise introduction of V element: For the first time, 0.01-0.02% V is added to 980MPa grade multiphase steel to form (Nb,Ti,V)C ternary nano-precipitates (size 5-15nm) with Nb and Ti. The strengthening effect is improved by 15-20% compared with traditional binary precipitates, and the average grain size is significantly refined to ≤5μm, which greatly improves the strength and toughness of the material.

[0017] (2) Si content optimization: The Si content is limited to 0.6-0.8%, which is 0.2% higher than the upper limit of the existing technology. This enhances the inhibition of cementite, promotes the enrichment of C in austenite, improves the stability of residual austenite by 25%, and improves the stress buffering capacity during the forming process.

[0018] (3) Mn-Cr synergistic regulation: By synergistically improving the hardenability of austenite, the transformation temperature range of bainite is widened to 15℃, reducing the difficulty of process control and improving the stability of product performance.

[0019] (4) Ultra-low S control: S≤0.005%, which is more than 90% lower than the existing technology, reduces MnS inclusions, eliminates the source of bending crack initiation, and enhances its forming performance.

[0020] (5) Control of residual austenite distribution: By precisely controlling the temperature at 695-705℃ during the continuous annealing and slow cooling stage, ferrite is promoted to grow epitaxially along the original austenite grain boundaries, thereby inducing the formation of residual austenite at the grain boundaries. When bending, the residual austenite at the grain boundaries undergoes the TRIP effect, which alleviates stress concentration.

[0021] (6) Segmented controlled cooling technology: The rapid cooling stage adopts a critical cooling rate of 28-32℃ / s, which avoids excessive martensite formation (≤15%) and ensures sufficient transformation of bainite (55-65%), thereby reducing the strength difference between phases and resolving the contradiction between strength and plasticity.

[0022] (7) Slow cooling process for billet: Slow cooling of continuous casting billet ≥ 48h reduces center segregation, makes Mn element segregation degree ≤ 1.2, and reduces hot rolling microstructure inhomogeneity;

[0023] (8) Precise temperature window for homogenization: 850-865℃ for incomplete austenitization, controlling the austenite volume fraction to 55-70%, laying the foundation for subsequent multiphase coordinated transformation, narrowing the temperature fluctuation range by 15℃ compared to existing technologies, and improving the stability of the microstructure.

[0024] The beneficial effects of the technical solution of this invention are as follows:

[0025] This invention innovatively adopts a technical system of "precise component ratio - multi-phase synergistic regulation - dynamic process matching": by introducing trace amounts of V elements to form composite nano-precipitates with Nb and Ti, combined with the microstructure regulation mechanism of "directional growth of epitaxial ferrite + gradient distribution of residual austenite" during continuous annealing, production can be achieved on a conventional pickling and rolling continuous annealing production line.

[0026] The multiphase steel provided by this invention has a tensile strength ≥980MPa, a yield strength ≥780MPa, an elongation after fracture (A50) ≥11%, and a hole expansion rate ≥45%. Moreover, it exhibits no cracks during transverse 0T cold bending. This invention solves the technical problems of existing 980MPa grade multiphase steel (CH980) lacking cold bending performance, being prone to cracking during the forming of complex parts, and having poor production stability. It can meet the stringent requirements of high-end automotive complex structural parts. Attached Figure Description

[0027] Figure 1 The metallographic structure of the multiphase steel in Example 1 is shown.

[0028] Figure 2 This is a SEM image of the multiphase steel from Example 1.

[0029] Figure 3 This is a diagram illustrating the effect of transverse 0T bending of multiphase steel in Example 1. Detailed Implementation

[0030] The following examples are intended to provide a better understanding of the technical solutions in this invention, but do not limit the invention in any way.

[0031] Examples 1-5

[0032] The chemical composition of the multiphase steels in each embodiment is shown in Table 1.

[0033] Table 1. Chemical composition (wt.%) of the multiphase steels in each embodiment.

[0034] 1 0.142 0.786 2.27 0.285 0.03 0.05 0.204 0.01 0.0128 0.005 2 0.157 0.628 2.40 0.281 0.031 0.044 0.181 0.016 0.0119 0.005 3 0.146 0.641 2.35 0.317 0.035 0.041 0.226 0.019 0.0116 0.003 4 0.152 0.716 2.49 0.295 0.039 0.042 0.213 0.013 0.0127 0.004 5 0.159 0.603 2.31 0.286 0.033 0.048 0.19 0.017 0.0125 0.003

[0035] The balance in Table 1 is Fe and unavoidable impurities.

[0036] The preparation methods of multiphase steel in each embodiment include the following steps:

[0037] 1) Smelting: According to the chemical composition shown in Table 1, the steel is smelted in a converter + refined by LF + refined by RH, and the [H] in the molten steel is controlled to be ≤2ppm and [O] ≤15ppm to obtain molten steel with the target composition.

[0038] 2) Continuous casting: Dynamic light reduction technology is adopted, the casting temperature is 1505-1515℃, the superheat of molten steel is 15-20℃, the thickness of the continuously cast billet is 210-240mm, and the slow cooling time of the billet is ≥48h. The continuous casting process parameters of each embodiment are shown in Table 2.

[0039] Table 2 Continuous casting process parameters for each embodiment

[0040] 1 1508 216 72 2 1509 227 60 3 1510 220 80 4 1512 238 72 5 1515 221 65

[0041] 3) Hot rolling: Cold billets are loaded into the furnace, heated to 1200-1250℃, and held in the furnace for 200-220 minutes; the initial rolling temperature for finishing rolling is 1050-1090℃, and the final rolling temperature is 910-930℃; laminar cooling adopts a front-stage rapid cooling + rear-stage slow cooling mode, with a coiling temperature of 590-610℃ and a 50℃ temperature compensation in the 100mm area at the beginning and end of the strip to ensure the uniformity of the longitudinal microstructure of the hot-rolled coil; the thickness of the hot-rolled coil is 2.6-3.3mm.

[0042] 4) Pickling and rolling: After the hot-rolled coil is pickled to remove the surface iron oxide scale, it is cold-rolled into a 1.2-1.6mm cold-rolled coil, with the cold rolling reduction rate controlled at 50-56%. The hot rolling and cold rolling process parameters for each embodiment are shown in Table 3.

[0043] Table 3 Hot rolling and cold rolling process parameters for each embodiment

[0044] 1 1235 200 1073 923 597 2.8 50% 2 1227 213 1065 919 610 2.7 55.5% 3 1248 206 1085 926 603 2.9 52% 4 1209 216 1058 911 607 3.0 53.3% 5 1237 219 1080 917 596 3.1 50%

[0045] 5) Continuous annealing: The chilled coil obtained above is slowly heated to a homogenization temperature of 850-865℃ and held for 160-180s; then slowly cooled to 695-705℃ at a cooling rate ≤5℃ / s; then rapidly cooled to 360-380℃ at a cooling rate of 28-32℃ / s; over-aging at 310-330℃ and held for 360-420s; finally cooled to below 80℃ for leveling, with a leveling elongation of 0.25-0.35%. The continuous casting annealing and leveling process parameters for each embodiment are shown in Table 4.

[0046] Table 4 Continuous annealing and leveling process parameters for each embodiment

[0047] 1 860 700 375 325 0.25 2 852 696 370 320 0.25 3 855 698 365 315 0.30 4 860 700 375 325 0.28 5 863 705 380 330 0.34

[0048] Example 1: Metallographic and SEM structures of multiphase steel as shown in Figure 1. Figure 1 and Figure 2 As shown, by Figure 1 and Figure 2 It can be seen that the microstructure of the multiphase steel in Example 1 consists of bainite, ferrite, martensite, and retained austenite. The microstructure proportions of the multiphase steels in each example are shown in Table 5.

[0049] Table 5. Microstructure of multiphase steel in each embodiment

[0050] 1 63.5 20.7 12.8 3.0 2 64.2 20.5 11.6 3.7 3 56.7 24.5 14.6 4.2 4 59.3 22.4 13.7 4.6 5 62.7 21.8 11.1 4.4

[0051] Example 1: Transverse 0T bending effect of multiphase steel as shown Figure 3 As shown, by Figure 3 It can be seen that the bending ridge of the bent specimen is parallel to the rolling direction, and there are no cracks after a 180° 0T bend at room temperature. The metallographic structure, SEM structure and 0T bending effect of the multiphase steel in the other examples are the same as those in Example 1, and will not be provided again.

[0052] The mechanical properties of the multiphase steels in each embodiment are shown in Table 6.

[0053] Table 6 Mechanical properties of multiphase steels in each embodiment

[0054] 1 799 1010 12.4 65 No cracks 2 792 1015 12.2 60 No cracks 3 789 1002 13.1 63 No cracks 4 790 1008 12.9 59 No cracks 5 802 1015 12.5 62 No cracks

[0055] As shown in Table 5, the multiphase materials provided by this invention all meet the performance requirements of tensile strength ≥980MPa, yield strength ≥780MPa, elongation after fracture A50 ≥11%, and porosity ≥45%, and there are no cracks when bent transversely at 0T.

[0056] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A 980MPa grade reinforced cold-rolled multiphase steel, characterized in that, The chemical composition of the multiphase steel, by mass percentage, is as follows: C: 0.14-0.16%, Si: 0.6-0.8%, Mn: 2.2-2.5%, P≤0.015%, S≤0.005%, Cr: 0.28-0.32%, Nb: 0.04-0.05%, Ti: 0.03-0.04%, Mo: 0.18-0.23%, V: 0.01-0.02%, with the remainder being Fe and unavoidable impurities.

2. The 980MPa grade reinforced cold-rolled multiphase steel according to claim 1, characterized in that, The microstructure of the multiphase steel consists of 55-65% bainite, 20-25% ferrite, 10-15% martensite and 3-5% retained austenite, with an average grain size ≤5μm.

3. The 980MPa grade reinforced cold-rolled multiphase steel according to any one of claims 1-3, characterized in that, The multiphase steel has a tensile strength ≥980MPa, a yield strength ≥780MPa, an elongation after fracture A50 ≥11%, a hole expansion rate ≥45%, and no cracks when cold-bent transversely.

4. The method for preparing 980MPa grade reinforced cold-rolled multiphase steel according to claim 1, characterized in that, This includes smelting, continuous casting, hot rolling, pickling, continuous annealing, and finishing processes; The continuous annealing process adopts a "gradient heating - precise uniform heating - segmented controlled cooling" process: heating to a uniform heating temperature of 850-865℃ and holding for 160-180s; slow cooling to 695-705℃ with a cooling rate ≤5℃ / s; rapid cooling to 360-380℃ with a cooling rate of 28-32℃ / s; over-aging temperature of 310-330℃ and holding for 360-420s; and final cooling to below 80℃.

5. The method for preparing 980MPa grade reinforced cold-rolled multiphase steel according to claim 4, characterized in that, The smelting process involves converter smelting, LF refining, and RH refining to control the molten steel's [H] ≤ 2ppm and [O] ≤ 15ppm, thereby obtaining molten steel with the target composition.

6. The method for preparing 980MPa grade reinforced cold-rolled multiphase steel according to claim 4, characterized in that, The continuous casting process employs dynamic light reduction technology, with a casting temperature of 1505-1515℃, a steel superheat of 15-20℃, a billet thickness of 210-240mm, and a billet slow cooling time of ≥48h.

7. The method for preparing 980MPa grade reinforced cold-rolled multiphase steel according to claim 4, characterized in that, The hot rolling process is as follows: cold billet is charged into the furnace, the heating temperature is 1200-1250℃, and the furnace time is 200-220 min; the finishing rolling temperature is 1050-1090℃, and the final rolling temperature is 910-930℃; the laminar flow cooling adopts the front-stage rapid cooling + rear-stage slow cooling mode, and the coiling temperature is 590-610℃.

8. The method for preparing 980MPa grade reinforced cold-rolled multiphase steel according to claim 4, characterized in that, The pickling and rolling process involves hot-rolled coils being pickled to remove surface iron oxide scale, followed by cold rolling, with the cold rolling reduction rate controlled at 53-56%.

9. The method for preparing 980MPa grade reinforced cold-rolled multiphase steel according to claim 4, characterized in that, The leveling process has a leveling elongation rate of 0.25-0.35%.

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