A cold-rolled 780-grade dh steel with fine grains and uniform structure and a production method thereof

Abstract

This invention relates to the field of high-strength cold-rolled steel sheet preparation technology, and particularly to a cold-rolled 780 grade DH steel with refined grains and uniform microstructure, and its production method. The chemical composition of the steel, by weight percentage, is: C: 0.08%–0.12%, Si: 0.4%–0.6%, Mn: 1.6%–2.0%, Nb: 0.02%–0.04%, P≤0.012%, S≤0.005%, Ti: 0.01%–0.02%, Als: 0.025%–0.050%, N≤0.0035%, with the balance being Fe and unavoidable impurities. The ferrite grain size in the steel of this invention is 5–8 μm, with a grain size standard deviation ≤1.2 μm, solving the problem of uneven microstructure. The finished steel has a tensile strength ≥780 MPa, elongation after fracture ≥22%, and earing rate ≤1.5%, meeting the requirements of complex deep drawing.

Description

Technical Field

[0001] This invention relates to the field of high-strength deep-drawing cold-rolled steel sheet preparation technology, and in particular to a cold-rolled 780 grade DH steel with refined grains and uniform microstructure and its production method. Background Technology

[0002] As a typical high-strength deep-drawing steel, cold-rolled 780 grade DH steel must simultaneously meet the core indicators of tensile strength ≥780MPa and elongation after fracture ≥20%. Its microstructure must be based on uniformly distributed fine-grained ferrite, supplemented by a small amount of dispersed precipitates (such as carbonitrides), to avoid formability fluctuations caused by coarse martensite or segregated inclusions.

[0003] The existing technology suffers from the following core pain points: First, the problem of coarse grains and uneven microstructure: Traditional processes rely on single rolling or conventional annealing, without forming a complete grain control system, resulting in finished product grain sizes mostly of 10-15μm, and significant differences in ferrite grain morphology (coarse grains larger than 20μm appear in some areas), which easily leads to cracking due to stress concentration during stamping; Second, the low utilization efficiency of microalloying elements: Existing technologies mostly add Nb or Ti elements alone, which easily forms coarse carbonitrides (such as NbC size > 50nm), which cannot effectively perform the functions of pinning grain boundaries and refining grain size. The effects of excessively high particle size actually reduce formability; thirdly, insufficient process coupling: hot rolling and continuous annealing process parameters are disconnected, for example, excessively high hot rolling final rolling temperature (>950℃) leads to austenite grain growth, making it difficult to completely refine in subsequent continuous annealing, resulting in a final product yield strength fluctuation range of ±50MPa, affecting the dimensional consistency of parts; fourthly, the significant impact of harmful elements: insufficient control precision of elements such as P, S, and N (e.g., P>0.018%), which easily segregate at grain boundaries to form brittle phases (e.g., Fe3P), exacerbating microstructure inhomogeneity and leading to "earing" defects during deep drawing (earing rate>3%). Existing technology 202311308190.9 "Continuous Annealing Method and Preparation Method of Duplex Steel with Enhanced Formability" mainly addresses the issues of hole expansion rate and bending performance, without explicitly mentioning the optimization effect on grain size and microstructure uniformity. Moreover, its focus is on continuous annealing and galvanizing, with insufficient synergistic optimization of the overall production process, limiting its production feasibility and adaptability to different scenarios, which is not conducive to large-scale promotion and application. The existing technology 202010928563.2, "A 780MPa grade high plasticity cold-rolled DH steel and its preparation method", adopts a common production process, but does not reflect the deep synergistic optimization of each process link, nor does it clearly show the advantages in cost control. It is difficult to achieve efficient and low-cost production while ensuring performance. It does not clearly explain the standard deviation of grain size and the degree of microstructure segregation, but only mentions the proportion of each phase, which is difficult to effectively avoid stress concentration and cracking problems during stamping.

[0004] In summary, existing technologies are insufficient to achieve the microstructure control goal of "fine grains + high uniformity". There is an urgent need to develop a whole-process coordinated grain refinement and microstructure homogenization technology to resolve the contradiction between high strength and deep drawing performance. Summary of the Invention

[0005] The core objective of this invention is to overcome the deficiencies of existing technologies and provide a cold-rolled 780 grade DH steel with refined grains and uniform microstructure, along with its production method. This invention differs from existing technologies that optimize only one stage; it solves the fragmentation problem in traditional grain control through the coordinated operation of parameters across the entire process, from composition design to molten iron purification, continuous casting, hot rolling, pickling, and continuous annealing. The synergistic effect of Nb-Ti composite microalloying and gradient cooling controls the precipitated phase size to 20-30 nm, significantly superior to existing technologies. This achieves stable control of the basalt grain size in the finished steel at 5-8 μm, with a grain size standard deviation ≤1.2 μm, resolving the issue of microstructure inhomogeneity. Through synergistic microalloying control and process coupling, the finished steel achieves a tensile strength ≥780 MPa, elongation after fracture ≥22%, and earing rate ≤1.5%, meeting the requirements of complex deep drawing.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A cold-rolled 780 grade DH steel with fine grains and uniform microstructure, wherein the chemical composition of the steel by weight percentage is: C: 0.08%~0.12%, Si: 0.4%~0.6%, Mn: 1.6%~2.0%, Nb: 0.02%~0.04%, P≤0.012%, S≤0.005%, Ti: 0.01%~0.02%, Als: 0.025%~0.050% (used for deoxidation and to avoid the formation of coarse Al2O3 inclusions), N≤0.0035%, with the balance being Fe and unavoidable impurities.

[0007] The steel has a yield strength of 380–420 MPa, a tensile strength of 780–820 MPa, and an elongation after fracture of 22–25%; the work hardening index n is 0.22–0.25.

[0008] The microstructure of the steel is as follows: ferrite grain size: 5–8 μm; martensite volume content: 15%–20%; grain size standard deviation ≤1.2 μm. Segregation degree ≤0.3 (determined according to GB / T15749-2008).

[0009] A method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure achieves grain refinement and microstructure homogenization through coordinated parameters across the entire process, including hot metal pretreatment, continuous casting, hot rolling, pickling, continuous annealing, and leveling. The method comprises the following steps: 1) Hot metal pretreatment: The three-stage purification process of "KR desulfurization + converter dephosphorization + RH vacuum degassing" is adopted to lay the foundation of purity for subsequent homogenization of the microstructure.

[0010] KR desulfurization: Add CaO-Al2O3-based desulfurizing agent to molten iron at a temperature of 1300-1350℃, with an addition amount of 3-5 kg / t iron. Use mechanical stirring at a speed of 60-80 r / min to ensure that the desulfurizing agent is in full contact with the molten iron. After desulfurization, S≤0.003%.

[0011] Converter dephosphorization: A "dual-slag method" is used for dephosphorization. The initial slag basicity is controlled at 2.0-2.5 (CaO / SiO2), and the final slag basicity is increased to 3.0-3.5. The oxygen blowing intensity is 1.2-1.5 Nm. 3 / (t・min), P≤0.010% after dephosphorization.

[0012] RH vacuum degassing: vacuum degree ≤67Pa, processing time 15~20min, removes N, H and other gases from molten steel, and finally N ≤0.0035% and H ≤2ppm in molten iron, avoiding structural defects caused by gas inclusions.

[0013] 2) Steelmaking continuous casting: The "low superheat casting + electromagnetic stirring + slow cooling treatment" are used to refine the grains of the continuous casting billet and avoid coarse columnar grains.

[0014] Converter steelmaking: The proportion of pretreated molten iron is 85% to 90%, the proportion of scrap steel is 10% to 15%, the blowing end temperature is 1620 to 1650℃, and Si-Mn alloy (0.8 to 1.0 kg / t steel), Nb-Fe alloy (0.2 to 0.4 kg / t steel), and Ti-Fe alloy (0.1 to 0.2 kg / t steel) are added during tapping to alloy the steel and ensure accurate composition.

[0015] LF refining: Heat to 1550~1580℃, add CaO-Al2O3-SiO2 refining slag, basicity 2.5~3.0, hold for 15~20min, adjust the composition uniformity so that the steel composition fluctuation is ≤±0.01% (C, Mn, Si). Continuous casting: Superheat is controlled at 15-25℃ (to avoid excessive superheat leading to coarse grains); the electromagnetic stirring current in the crystallizer is 200-250A, and the frequency is 2-3Hz; columnar crystal growth is suppressed, and equiaxed crystal formation is promoted; billet drawing speed: 1.0-1.2m / min (matching slab cross-section 200mm×1500mm); after the billet exits the crystallizer, segmented water spray gradient slow cooling is adopted, and the cooling rate is gradually reduced by controlling the water spray intensity in different zones, with a cooling rate of 5-8℃ / min; to avoid microstructure segregation caused by internal stress, the grain size of the continuously cast billet is controlled at 50-80μm, laying the foundation for subsequent hot rolling refinement.

[0016] 3) Hot rolling process: The hot-rolled plate grains are refined by a combination of "low-temperature heating + multi-pass large reduction + controlled rolling + laminar gradient cooling" to provide fine-grained billets for cold rolling.

[0017] The continuously cast billet is fed into the heating furnace and "segmented heating" is adopted. The preheating section of the heating furnace is 600-800℃ and held for 60-90 minutes. The heating section is 1180-1220℃ (lower than the traditional 1250℃ to avoid excessive growth of austenite grains) and held for 120-150 minutes to ensure that Nb and Ti elements are completely dissolved.

[0018] The roughing process uses a reversible roughing mill, with 5 to 7 passes, and a total reduction of 84% to 88%. The pass reduction is as follows: 20% to 25% for passes 1 to 2, 15% to 20% for passes 3 to 5, and 10% to 15% for passes 6 to 7. The total reduction is calculated as (thickness before rolling - thickness after rolling) / thickness before rolling × 100%. The pass reduction is calculated as (entry thickness of this pass - exit thickness of this pass) / entry thickness of this pass × 100%.

[0019] Roughing mill exit temperature: 1050~1080℃, to avoid excessively low temperature causing a sudden increase in rolling force.

[0020] The finishing mill consists of 7 stands of continuous finishing mill, using "controlled rolling". The final rolling temperature is 880-920℃ (50-80℃ above Ar3 to ensure no austenite phase transformation). The total reduction rate of finishing mill is 87%-94%, and the reduction rate of the last stand is ≥18% (accumulating dislocations through large deformation to promote subsequent phase transformation and refine grains). The rolling speed is 10-12 m / s to ensure uniform deformation.

[0021] Laminar flow cooling employs a gradient cooling method of "rapid cooling in the front section + slow cooling in the back section": the cooling rate in the front section is 25-35℃ / s, rapidly cooling to 750-780℃, below Ar3, to induce ferrite phase transformation; the cooling rate in the back section is 5-10℃ / s, cooling to 550-600℃ for coiling, promoting the dispersed precipitation of Nb(C,N), pinning ferrite grains, and avoiding coarsening of precipitates due to excessively high coiling temperature.

[0022] The thickness of the hot-rolled sheet is 2.0 to 4.5 mm, and the grain size is controlled at 12 to 15 μm (18 to 22 μm in traditional processes).

[0023] 4) Pickling and rolling process: "High-efficiency pickling + constant pressure cold rolling" is used to remove iron oxide scale and accumulate deformation energy, laying the foundation for continuous annealing and recrystallization. Pickling adopts "continuous hydrochloric acid pickling" with the following parameters: hydrochloric acid concentration: 18%~22%; pickling temperature: 60~70℃; pickling time: 3~5min (adjusted according to the thickness of iron oxide scale).

[0024] Rinsing section: A 3-stage countercurrent rinsing process is adopted to ensure that the residual Cl⁻ on the strip surface is ≤50mg / m². 2 To avoid tissue defects caused by corrosion.

[0025] Cold rolling: 6-stand continuous cold rolling mill, using "constant pressure rolling" to control deformation uniformity; total cold rolling reduction: 60-70%; Pass reduction ratio allocation: 25-30% for the first stand, 20-25% for the second and third stands, and 15-20% for the fourth to sixth stands; Total reduction ratio = (thickness before rolling - thickness after rolling) / thickness before rolling × 100%; Pass reduction ratio = (entry thickness of this pass - exit thickness of this pass) / entry thickness of this pass × 100%.

[0026] The rolling force is dynamically adjusted according to the strip material, with a fluctuation range of ≤±5%; rolling speed: 800~1000m / min.

[0027] The finished thickness of the cold-rolled sheet is 1.0 to 1.8 mm, and the deformation energy storage is ≥200 J / mol (verified by DSC test), providing sufficient energy for continuous annealing and recrystallization.

[0028] 5) Continuous Annealing Process: The process employs "rapid heating + critical zone annealing + segmented cooling + precise leveling" to achieve further grain refinement and microstructure homogenization of cold-rolled steel sheets. Continuous Annealing (CAL): A vertical continuous annealing furnace is used, with five stages: preheating stage, heating stage, first-stage cooling, second-stage cooling, and over-aging stage.

[0029] Preheating section: room temperature → 500℃, heating rate 50~60℃ / s (rapid heating reduces grain growth time); Heating section: 500℃ → 780~820℃ (critical temperature, ferrite + austenite dual-phase region), holding for 30~40s (ensuring uniform austenite nucleation and avoiding localized grain coarsening); Cooling section: First cooling stage: 780~820℃ → 650~680℃, cooling rate 15~20℃ / s (promoting austenite to ferrite transformation); Second cooling stage: 650~680℃ → 450~500℃, cooling rate 5~8℃ / s (promoting secondary precipitation of Nb(C,N) and further pinning grains); Over-aging section: 450~500℃, holding for 20~30s (eliminating internal stress and stabilizing the microstructure); Grain size of the continuous annealing plate: 5~8μm, grain size standard deviation ≤1.0μm.

[0030] By controlling the hot rolling final rolling temperature to 880–920℃ and the cooling rate of the first stage of continuous annealing to 15–20℃ / s, the martensite content in the steel is made to be 15%–20%.

[0031] 6) The leveling process uses a double-frame leveling machine with "small elongation + constant tension" control: Leveling elongation: 0.2%~0.3% (to avoid excessive leveling leading to work hardening and affecting formability); Leveling tension: 15~20kN (adjusted according to the strip width to ensure the flatness of the strip is ≤2I); Work roll roughness: Ra=0.8~1.2μm (to ensure the surface quality of the strip and meet the requirements of deep drawing).

[0032] This invention employs a "C-Mn-Si+Nb-Ti composite microalloying" system, precisely controlling the element ratios to synergistically refine grains. The chemical composition range and design rationale are as follows: C: 0.08~0.12%, controls austenite stability and avoids excessive martensite formation; at the same time, it provides a C source for Nb / Ti carbonitrides and ensures the dispersion of precipitated phases.

[0033] Mn: 1.6~2.0%, improves hardenability, promotes deformation-induced precipitation during hot rolling, and inhibits austenite grain growth.

[0034] Si: 0.4-0.6%, solid solution strengthens ferrite while inhibiting cementite precipitation, ensuring deep drawing performance.

[0035] Nb: 0.02-0.04%, forms nanoscale Nb(C,N) precipitates with C and N, pinning grain boundaries and hindering grain growth; during the hot rolling stage, precipitation is induced by deformation, refining austenite grains.

[0036] Ti: 0.01-0.02%, combines with excess N to form TiN, avoiding preferential reaction between Nb and N, and ensuring effective precipitation of Nb(C,N); at the same time, TiN can act as a nucleation core to refine the grains of the continuously cast billet.

[0037] P≤0.012%, S≤0.005%, N≤0.004%, strictly control harmful elements, reduce grain boundary segregation and brittle inclusions, and improve the uniformity of the microstructure.

[0038] Als: 0.025%~0.050%, used for deoxidation and to avoid the formation of coarse Al2O3 inclusions.

[0039] Compared with the prior art, the beneficial effects of the present invention are: 1. Significantly improved microstructure uniformity: The grain size of the finished steel rib is refined from the traditional 10-15μm to 5-8μm, the standard deviation of grain size is ≤1.2μm, and the microstructure segregation is reduced by more than 60%, effectively avoiding stress concentration and cracking during stamping.

[0040] 2. Synergistic optimization of mechanical properties: The finished steel has a tensile strength of 780-820MPa, a yield strength of 380-420MPa, an elongation after fracture of 22-25%, and an n-value (work hardening index) of 0.22-0.25, meeting the highest grade requirements of 780 grade DH steel in GB / T2518-2019.

[0041] 3. Excellent deep-drawing performance: Ear-making rate ≤1.5% (3-5% in traditional processes). In stamping tests of automotive body panels (such as door inner panels), the pass rate has increased from 85% to over 98%, and the forming stability has been significantly improved.

[0042] 4. High production feasibility: Based on the existing cold rolling production line, only process parameters (such as heating temperature and cooling rate) need to be optimized. No new equipment is required, and the cost per ton of steel is reduced by 15 to 20 yuan, which is suitable for large-scale production.

[0043] 5. Controllable composition cost: By adopting Nb-Ti composite microalloying (total addition ≤0.06%), compared with the addition of Nb alone (0.08~0.10%), the cost of microalloying is reduced by more than 30%, balancing performance and economy. Attached Figure Description

[0044] Figure 1 This is a 500x metallographic microstructure image corresponding to Example 1. Detailed Implementation

[0045] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto. Experimental methods for which specific conditions are not specified in the embodiments are generally determined according to national / industry standards; if there is no corresponding national / industry standard, then they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0046] This invention discloses a cold-rolled 780 grade DH steel with refined grains and uniform microstructure, and its production method. Grain refinement and microstructure uniformity are achieved through "precise composition design + coordinated control of the entire process." It is suitable for applications with stringent requirements for formability and dimensional stability, such as automotive body panels and household appliance structural components, and enables coordinated control of high strength and excellent deep-drawing performance.

[0047] The chemical composition and mass percentage of the examples are shown in Table 1; the smelting process of the examples is shown in Table 2; the hot rolling process of the examples is shown in Table 3; the pickling rolling process of the examples is shown in Table 4; the continuous annealing process of the examples is shown in Table 5; the leveling process of the examples is shown in Table 6; the mechanical properties and microstructure of the examples are shown in Table 7; and the comparison between the examples and traditional processes is shown in Table 8. Example 1 is used for stamping the inner panel of an automobile door, with a stamping speed of 30 mm / s, a pass rate of 98.5%, and no cracking defects; Example 2 is used for stamping the inner drum of a washing machine, with a forming depth of 150 mm, a pass rate of 99%, and a dimensional tolerance of ±0.1 mm, meeting high precision requirements; Example 3 is used for stamping the outer panel of an automobile trunk lid, with a surface quality grade reaching Grade I in GB / T13237-2023, no scratches or dents, and a pass rate of 98%.

[0048] Table 1 Chemical composition of the examples (%): Table 2 Smelting process parameters for the examples: Table 3 Hot rolling process parameters for the examples: Table 4. Pickling and rolling process parameters for the examples: Table 5. Continuous annealing process parameters for the embodiments: Table 6. Leveling process parameters for the embodiments: Table 7 Mechanical properties and microstructure of the examples: Table 8 Comparison of the Examples with Traditional Processes: like Figure 1 The image shows the 500x metallographic microstructure corresponding to Example 1. Samples were prepared according to GB / T13298-2015, using a metallographic cutting machine with water cooling, clamping, and grinding with 180#, 320#, 600#, and 800# sandpaper. They were then coarsely polished with a 3.5μm polishing agent, followed by a 0.5μm fine polishing, and etched with 4% nitric acid alcohol for 4 seconds. The average grain size level of the metal is assessed according to GB / T6394-2017; the non-metallic inclusion level in steel is assessed according to GB / T10561-2023; and the second phase area content is measured according to GB / T18876.1-2002. Table 8 shows the corresponding grain size, inclusion level, and other data.

[0049] Table 9 shows the information such as the isogranular size corresponding to Example 1. The microstructure design of this invention is "ferrite (F) + martensite (M) + martensite-austenite mixed structure (MA)". The martensite ratio is controlled at 15-20%, which can synergistically ensure strength and plasticity. Therefore, the martensite ratio is clearly defined.

[0050] Table 9: Depend on Figure 1 The 500x metallographic microstructure shown indicates that the steel has a continuous ferrite matrix, appearing light gray in the OM field of view. The grain outlines are clear and the morphology is relatively regular, without grain elongation, orientation segregation, or localized coarsening caused by uneven rolling deformation. Image-Pro-Plus image analysis software was used to analyze five non-overlapping fields of view (each field of view area ≥ 0.1 mm²). 2Statistics show that the average ferrite grain size is 6.2 μm, the standard deviation of grain size is only 0.9 μm, and the size distribution is concentrated in 5.5-7.0 μm, with no coarse grains >8 μm. This grain refinement effect is significantly better than the traditional process (ferrite grain size 10-15 μm, standard deviation ≥2.0 μm), directly confirming the effectiveness of the present invention "Nb-Ti composite microalloying + full-process synergy": Ti element preferentially combines with N to form TiN, which acts as the nucleation core of the continuously cast billet grains and inhibits the growth of columnar crystals; Nb element induces the precipitation of nano-sized Nb(C,N) through deformation during the hot rolling stage, pinning the austenite grain boundaries and hindering grain growth. The two work together to achieve precise refinement of ferrite grains.

[0051] Martensite appears dark gray in the OM field of view, uniformly distributed in discontinuous thin strips or small islands at the ferrite grain boundaries and within the grains, without agglomeration, clustering, or local enrichment. Image-Pro-Plus area fraction analysis shows a martensite proportion of 18%. This proportion is precisely controlled through the process parameters of this invention: the hot rolling final rolling temperature is controlled at 900℃ (50-80℃ above Ar3) to ensure that austenite does not undergo phase transformation and retains sufficient deformation energy; the cooling rate in the first stage of continuous annealing is set at 18℃ / s to promote the uniform transformation of austenite to martensite—meeting the strength requirement of tensile strength ≥800MPa while avoiding the plasticity deterioration caused by excessive martensite (22-25%) in traditional processes. This provides structural support for the excellent mechanical properties of "24% elongation after fracture and work hardening index n=0.24" in Example 1.

[0052] A trace amount of light gray MA islands (martensite-austenite mixed structure) with a size of about 0.5-0.8 μm can be observed at the martensite / ferrite interface. They appear "semi-transparent" under OM (Oxygen Oxide) and show a clear difference in grayscale from ferrite and martensite. X-ray diffraction (XRD) tests further confirmed that the content of retained austenite in the MA islands is 2-3%. This retained austenite can undergo phase transformation-induced plasticity (TRIP effect) during deep drawing, absorbing deformation energy to alleviate stress concentration. This is consistent with the application effect of Example 1, which shows that "the stamping pass rate of automobile door inner panel is 98.5% and there are no crack defects", demonstrating the directional optimization effect of the microstructure design of this invention on deep drawing performance.

[0053] It should be clarified that the yield strength design (380-420 MPa) of the cold-rolled 780 grade DH steel in this invention is not "too low," but rather an optimal range formed based on the core requirement of DH steel's "synergy between high strength and deep drawing formability," combined with material mechanics mechanisms and industrial application scenarios. During deep drawing (such as the drawing of automotive body panels), the material needs to undergo large plastic deformation. A yield strength of 380-420 MPa can avoid "excessive stamping force" (such as exceeding the rated load of the stamping equipment) caused by excessively high yield strength, while reducing the springback after part forming (springback rate ≤2%, compared to ≥5% for traditional high-yield steel), thus improving dimensional accuracy.

Claims

1. A cold-rolled 780 grade DH steel with refined grains and uniform microstructure, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.08%–0.12%, Si: 0.4%–0.6%, Mn: 1.6%–2.0%, Nb: 0.02%–0.04%, P≤0.012%, S≤0.005%, Ti: 0.01%–0.02%, Als: 0.025%–0.050%, N≤0.0035%, with the balance being Fe and unavoidable impurities.

2. The cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 1, characterized in that, The yield strength is 380–420 MPa, the tensile strength is 780–820 MPa, the elongation after fracture is 22%–25%, and the work hardening index n is 0.22–0.

25.

3. The cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 1, characterized in that, The microstructure of the steel is as follows: ferrite grain size is 5-8 μm; martensite volume content is 15%-20%; grain size standard deviation is ≤1.2 μm; and microstructure segregation is ≤0.

3.

4. A method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure as described in any one of claims 1-3, characterized in that, Specifically, the methods and steps are as follows: 1) Hot metal pretreatment includes: KR desulfurization: Adding CaO-Al2O3-based desulfurizing agent to molten iron at a temperature of 1300-1350℃ for desulfurization; Converter dephosphorization: The "double slag method" is adopted for dephosphorization, with the initial slag basicity controlled at 2.0-2.5 and the final slag basicity increased to 3.0-3.5; RH vacuum degassing: vacuum degree ≤67Pa, processing time 15~20min, final molten iron N≤0.0035%, H≤2ppm; 2) Steelmaking continuous casting includes: The superheat of continuous casting is controlled at 15-25℃; the electromagnetic stirring current of the crystallizer is 200-250A, the frequency is 2-3Hz; and the cooling rate is 5-8℃ / min. 3) Hot rolling process includes: Heating furnace heating section: 1180~1220℃, holding temperature 120~150min; roughing rolling total reduction rate 84%~88%; roughing rolling exit temperature: 1050~1080℃; finishing rolling final rolling temperature: 880~920℃; finishing rolling total reduction rate: 87%~94%, last stand reduction rate ≥18%; Laminar flow cooling front section cooling rate: 25~35℃ / s, rapidly cooling to 750~780℃; rear section cooling rate: 5~10℃ / s, cooling to 550~600℃ for winding. 4) Pickling and rolling process includes: total cold rolling reduction rate: 60-70%; 5) Continuous annealing process includes: Preheating section: room temperature → 500℃, heating rate 50~60℃ / s; heating section: 500℃ → 780~820℃, holding for 30~40s; Cooling section: First cooling stage: 780~820℃→650~680℃, cooling rate 15~20℃ / s; Second cooling stage: 650~680℃→450~500℃, cooling rate 5~8℃ / s; Over-aging stage: 450~500℃, holding for 20~30s.

5. The method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 4, characterized in that, The grain size of the continuously cast billet is controlled at 50–80 μm.

6. The method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 4, characterized in that, The finishing rolling speed is 10-12 m / s.

7. The method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 4, characterized in that, The thickness of the hot-rolled sheet is 2.0 to 4.5 mm, and the grain size is controlled between 12 and 15 μm.

8. The method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 4, characterized in that, In the pickling and rolling process, the concentration of hydrochloric acid in the pickling solution is 18%–22%; the pickling temperature is 60–70℃; the pickling time is 3–5 min; and the cold rolling speed is 800–1000 m / min.

9. The method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 4, characterized in that, Cold-rolled sheet finished thickness: 1.0~1.8mm, deformation energy storage ≥200J / mol.

10. The method for producing cold-rolled 780 grade DH steel with refined grains and uniform microstructure according to claim 4, characterized in that, It also includes leveling process: leveling elongation: 0.2%~0.3%; leveling tension: 15~20kN; work roll roughness: Ra=0.8~1.2μm.

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