Production method of hot-rolled and hot-formed steel based on endless continuous casting and rolling production line

Abstract

The invention relates to the technical field of metallurgy, and particularly discloses a production method of hot-rolled hot-formed steel based on a full endless continuous casting and rolling production line, which comprises the following steps: adjusting the contents of C, Mn, Cr, Al and B elements, carrying out continuous casting production on an upper limit steel billet which relimits the contents of P, N and S elements, and setting continuous casting process parameters; continuously-cast molten steel enters an induction heating furnace after being formed through a tundish and a crystallizer; the steel billet enters a roughing mill group of an ESP production line to be rolled and then enters a finishing mill group to be rolled; applying a hot pressing process and an electric welding process to the ESP hot-formed steel, and setting parameters; on the basis of an ESP short-process production line, the 1500Mpa-grade short-process hot forming steel is provided, the production procedures are reduced, the production cost is reduced, and the quenching forming surface quality of the hot forming steel is improved while the performance requirements of the hot forming steel for automobiles are met.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line. Background Technology

[0002] The current manufacturing process for hot-formed steel used in automobiles mainly involves "traditional thick plate continuous casting + hot rolling + pickling + cold rolling + heat treatment," which is lengthy, complex, and results in high carbon emissions, failing to meet the low-carbon development needs of the automotive industry. How to reduce carbon emissions during the production of automotive steel through process reform is an important future development direction.

[0003] Driven by the automotive industry's dual goals of lightweighting and high safety, hot-formed steel, with its superior characteristics of high strength and low weight, has become a core material for passenger vehicle body structural components. Industry data shows that the current application rate of hot-formed steel in passenger vehicle bodies has climbed to 14%, significantly improving the passive safety performance of the vehicle body. However, its high production and application costs have become a key bottleneck restricting the large-scale application of automotive lightweighting technology. Under the traditional cold rolling process, the production process of 1.0-2.0mm thick hot-formed steel is not only lengthy, but also suffers from problems such as secondary sandblasting and part scrapping due to excessive high-temperature oxidation during hot stamping, which exacerbates the material cost pressure.

[0004] Production process innovation offers a new approach to breaking this deadlock. Compared to traditional cold rolling processes, short-process production lines are capable of producing ultra-thin 0.75mm sheets. If applied to hot-formed steel production, cold rolling and annealing processes can be eliminated, significantly reducing production costs. However, hot-rolled products inherently suffer from poor microstructure and composition uniformity, leading to significant performance fluctuations during quenching. Simultaneously, the 900-970℃ high temperatures during hot stamping cause surface oxidation, forming excessively thick and uneven iron oxide scale. This not only increases sandblasting costs but also causes the scale falling into the molds, abrading them and affecting mold life. Therefore, introducing short-process production lines into hot-formed steel production and improving the quenched surface quality of hot-formed steel will be the core breakthrough in reducing the overall cost of using hot-formed steel.

[0005] Furthermore, with the surge in demand for thin-gauge (≤1.4mm) hot-formed steel due to automotive lightweighting, short-process hot-formed steel production faces challenges such as mill overload caused by complex alloy compositions and high thermal strength. These technical contradictions urgently necessitate an innovative solution that balances cost control, quality improvement, and production feasibility.

[0006] The fully automated continuous casting and rolling (ESP) production line, a landmark technology of the "Third Technological Revolution" in the steel industry, achieves a high degree of integration and continuity in the production process through the rigid connection of continuous casting and rolling processes. With its advanced automated control system and optimized production line configuration, it can significantly reduce energy consumption and carbon emissions, improve yield, and achieve high-precision production of thin-gauge strip steel. Against this backdrop, developing hot-formed steel for automobiles based on the ESP production line is of great significance for promoting the lightweighting of automobiles and implementing the energy conservation and emission reduction strategy of the steel industry. Therefore, it is necessary to design a production method for hot-rolled hot-formed steel based on the fully automated continuous casting and rolling production line. Summary of the Invention

[0007] To address the problems existing in the prior art, the purpose of this invention is to provide a method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line.

[0008] The technical solution adopted by this invention to solve its technical problem is: a method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line, comprising the following steps:

[0009] The chemical composition of S1 hot-formed steel by mass percentage is as follows: C: 0.21-0.26%, Mn: 1.20-1.80%, Si: 0.20-0.35%, S≤0.003%, P≤0.015%, Cr: 0.20-0.47%, Alt: 0.04-0.07%, N≤0.005%, Ti: 0.045-0.055%, B: 0.0025-0.0035%, with the remainder being iron and unavoidable impurities;

[0010] S2. The steel billet is continuously cast, and the continuous casting process parameters are set.

[0011] S3. After being formed by the tundish and crystallizer, the continuously cast molten steel enters the induction heating furnace.

[0012] S4. The steel billet enters the roughing mill of the ESP production line for rolling, and then enters the finishing mill for rolling.

[0013] The application of hot pressing and electric welding processes for S5 and ESP hot-formed steels and the setting of parameters.

[0014] Specifically, the continuous casting process parameters in step S2 are set as follows: superheat 15-25℃, N content increase during casting ≤3ppm, AlS loss during casting ≤40ppm, target casting speed control is 4.5m / min during mixed casting and 5.0m / min after mixed casting, standard deviation of liquid level fluctuation in the crystallizer ≤1.5mm, and temperature difference of the billet cross section ≤30℃.

[0015] Specifically, in step S3, the molten steel in the continuous casting is baked in the tundish for ≥2.0h at a temperature ≥1200℃, the immersion nozzle in the tundish is baked for ≥1.5h at a temperature ≥1200℃, and the crystallizer taper is 1.14-1.26%.

[0016] Specifically, the process control requirements for the induction heating furnace in step S3 are an outlet temperature of 1150-1170℃ and a heating temperature controlled at the upper limit of 1250℃.

[0017] Specifically, in step S4, the roughing mill of the ESP production line adopts R1 mill and R3 mill. The inlet temperature difference of R1 mill is ≤50℃, and the outlet temperature of R3 mill is >1000℃.

[0018] Specifically, in step S4, the ESP production line sets the yield strength, tensile strength, and elongation of the billet raw material according to the different thickness specifications of the hot-formed steel. The thickness specification of the hot-formed steel is 1.2-1.6mm, the yield strength of the billet raw material is 441-467MPa, the tensile strength of the billet raw material is 615-632MPa, and the elongation of the billet raw material is 20.0-22.0%.

[0019] Specifically, in step S4, the final rolling temperature of the finishing mill is controlled at 830±20℃, and the coiling temperature is controlled at 550±20℃.

[0020] Specifically, the hot pressing process of the ESP hot-formed steel in step S5 uses a heating temperature of 900-950℃ and a heating time of the thickness of the hot-formed steel * 100 + 120s.

[0021] Specifically, the spot welding process of ESP hot-formed steel in step S5 has a welding current range of ≥1.0kA under the specified welding time conditions.

[0022] The present invention has the following beneficial effects:

[0023] This invention presents a production method for hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line. Based on the ESP short-process production line, it proposes a short-process hot-formed steel production process for 1500Mpa level steel, reducing production steps and lowering production costs. While meeting the performance requirements of automotive hot-formed steel, it also improves the surface quality of hot-formed steel after quenching. Attached Figure Description

[0024] Figure 1 Engineering stress-strain curves for hydrogen-charged tensile tests on ESP thermoformed samples.

[0025] Figure 2 Engineering stress-strain curves for hydrogen-charged tensile tests on cold-rolled samples.

[0026] Figure 3 A line graph showing the welding current at a specified welding time point.

[0027] Figure 4 Comparison diagrams showing the trial molding verification of commonly used parts for body-in-white.

[0028] Figure 5 Comparison of surface conditions after hot stamping trials of different embodiments of ESP hot-formed steel.

[0029] Figure 6 Comparison of surface conditions of ESP hot-formed steel at different coiling temperatures. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0031] A production method for hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line, which adjusts the content of C, Mn, Cr, Al and B elements on the basis of 22MnB5, and re-limits the upper limit of the content of P, N and S elements.

[0032] The chemical composition of hot-formed steel by mass percentage is as follows: C: 0.21-0.26%, Mn: 1.20-1.80%, Si: 0.20-0.35%, S≤0.003%, P≤0.015%, Cr: 0.20-0.47%, Alt: 0.04-0.07%, N≤0.005%, Ti: 0.045-0.055%, B: 0.0025-0.0035%, with the remainder being iron and unavoidable impurities;

[0033] The adjustment of alloying elements in this invention is mainly based on the following principles:

[0034] (1) Mn: Manganese is a key element to ensure hardenability. Since ESP hot-formed steel does not have a subsequent cold rolling annealing process, it is impossible to ensure the uniformity of the material composition and structure. The segregation of Mn is particularly serious. This micro-uniformity of composition and structure will eventually be inherited by the quenched parts and cause the performance of the parts to be inconsistent. Therefore, in order to compensate for this compositional non-uniformity, the content of Mn is appropriately increased in this invention to ensure the overall quenching uniformity. However, the large increase of Mn will cause difficulties in ESP production. In addition, it will also aggravate oxidation and damage the surface quality of the hot-pressed parts. To balance the two, this invention limits the Mn content to this range.

[0035] (2) Al: Al plays a deoxidizing role in the steelmaking process. At the same time, it combines with N to form AlN, which can refine the austenite grains and has a positive effect on the uniformity of the structure after quenching. It also consumes the N content in the steel, thereby increasing the effective B content, which can also indirectly improve the hardenability of the material. Therefore, the present invention appropriately increases the Al content.

[0036] (3) Cr: Cr is another key factor in ensuring the hardenability of steel. In addition to ensuring hardenability, Cr is also a key factor in ensuring the surface quality of hot-formed steel. During the quenching process, Cr can agglomerate on the surface layer of the material and selectively oxidize to form a protective film, which slows down the oxidation process of the steel matrix. At the same time, Cr element inhibits the diffusion of C and also plays a role in reducing the thickness of the surface decarburized layer. Therefore, as the core element to improve the hardenability and surface quality of the material, the present invention appropriately increases the Cr content.

[0037] (4) S: The core of the headless continuous casting and rolling production line is high-speed continuous casting. High-purity molten steel is the key to ensuring high casting speed in ESP continuous casting. Excessive S content can easily cause defects in the billet during ESP continuous casting and can also easily cause billet cracks during hot rolling, affecting the smooth operation of production. Therefore, this invention controls the S content to ≤0.002%.

[0038] (5) P: ESP hot-rolled hot-formed steel has a high alloy content, which increases the risk of cracking in application. As a key factor causing cracking, reducing its content will reduce the risk of cracking in application. Therefore, the present invention controls the P content to ≤0.015%.

[0039] (6) N: Nitrogen will precipitate high-melting-point precipitates such as TiN and BN in steel, which will reduce the effective content of the alloy. In addition, coarse TiN particles will seriously damage the plasticity of steel. Therefore, the present invention controls the nitrogen content to ≤0.004% without increasing the process cost.

[0040] A low-cost, high-performance, high-quenching-surface-quality hot-formed steel for automobiles based on a fully headless continuous casting and rolling production line.

[0041] I. The specific requirements for the ESP rolling process are as follows:

[0042] 1. The steel billet is produced by continuous casting, and the continuous casting process parameters are set as follows: superheat 15-25℃, N content increase during casting ≤3ppm, AlS loss during casting ≤40ppm, target casting speed control is 4.5m / min during mixed casting and 5.0m / min after mixed casting, standard deviation of liquid level fluctuation in the crystallizer ≤1.5mm, and temperature difference of billet cross section ≤30℃.

[0043] 2. After being formed in the tundish and crystallizer, the continuously cast molten steel enters the induction heating furnace. The tundish baking time for the continuously cast molten steel is ≥2.0h, and the baking temperature is ≥1200℃. The baking time for the immersion nozzle in the tundish is ≥1.5h, and the baking temperature is ≥1200℃. The crystallizer taper is 1.14-1.26%. The process control requirements for the induction heating furnace are an outlet temperature of 1150-1170℃ and a heating temperature controlled at the upper limit of 1250℃.

[0044] 3. The steel billet enters the roughing mill of the ESP production line for rolling, and then enters the finishing mill for rolling.

[0045] The roughing mills of the ESP production line use R1 and R3 mills. The inlet temperature difference of the R1 mill is ≤50℃, and the outlet temperature of the R3 mill is >1000℃.

[0046] The finishing mill's final rolling temperature is controlled at 830±20℃, and the coiling temperature is controlled at 550±20℃.

[0047] ESP hot-rolled hot-formed steel, prepared using the above composition and process, was compared with cold-rolled hot-formed steel in terms of raw material and plate quenching properties. The comparative properties are as follows:

[0048] (1) Raw material properties

[0049] As shown in Table 1, tensile properties were tested at the edge, middle, and 1 / 4 position of the plate. This steel grade significantly reduced the yield strength of the raw material and the yield strength ratio of the material.

[0050] Table 1 Comparison of raw material properties

[0051]

[0052] (2) Plate quenching performance

[0053] Table 2 shows the properties of the material after plate quenching. The plate quenching process is as follows: 950℃, holding temperature for 6 min, load 15KN, cooling time 20s, and demolding temperature 50-100℃.

[0054] Table 2 Properties of materials after plate quenching

[0055]

[0056] (3) Resistance to hydrogen embrittlement

[0057] like Figures 1-2 As shown, hydrogen-charged tensile tests were conducted on ESP hot-formed and cold-rolled samples with a thickness of 1.5 mm. The reduction in elongation before and after hydrogen charging was calculated to reflect the material's resistance to hydrogen embrittlement. The test results showed that the hydrogen embrittlement sensitivity of the hot-rolled ESP material was 44.9%, while that of the cold-rolled material was 41.6%, which were similar.

[0058] The hydrogen embrittlement resistance of ESP hot-formed steel after plate quenching was tested according to standard T / CSAE 155-2020 "Test Method for Hydrogen-Induced Delayed Fracture Sensitivity of Ultra-High Strength Automotive Steel Sheets under U-Shaped Constant Bending Load". Pre-bending loads at different stress levels were applied to the samples, and all pre-bent samples were immersed in a simulated hydrogen environment solution. The hydrogen environment solution was prepared as a 0.1 mol / L hydrochloric acid solution, and the immersion time was 300 h.

[0059] II. Hot-pressing Application Process of ESP Hot-Formed Steel

[0060] 1. Hot pressing process parameters

[0061] The hot pressing process window for ESP hot-formed steel is shown in Table 3:

[0062] Table 3 Hot pressing process window for ESP hot-formed steel

[0063] project Require Heating temperature / ℃ 900-950℃ Heating time / s Thickness * 100 + 120

[0064] Other parameters for hot pressing, shot blasting, and laser engraving are determined based on the specific parts and can be replaced by equivalent parameters for cold rolling.

[0065] 2. Spot welding process parameters

[0066] The welding current range of this invention is as follows: Figure 3 As shown in Table 4, the welding current range corresponding to the specified welding time point is ≥1.0kA.

[0067] Table 4 Welding current corresponding to specified welding time points

[0068]

[0069] III. Molding Properties

[0070] Trial molding verification was conducted on commonly used components of the body-in-white. The trial molded parts included B-pillar reinforcement plates, A-pillar reinforcement plates, A-pillar inner panels, and door anti-collision bars. The trial results are as follows: Figure 4 As shown, the parts are well-formed, with no severe iron oxide scale on the surface, no cracks or hidden cracks, and the dimensional accuracy meets the requirements.

[0071] Table 5 shows the results of partial sample testing for properties such as thinning rate, hardness, and static tensile strength.

[0072] Table 5 Comparison of Thinning Rate, Hardness, and Static Tensile Performance of Samples Taken Partially

[0073] Trial mold parts Tensile properties Vickers hardness / HV10 Thinning rate Rear end of front longitudinal beam Yield strength: 1005-1128 MPa; Tensile strength: 1505-1597 MPa; Elongation (A50): 6.5%-8.0% 416-501 5% Rear longitudinal beam front end Yield strength: 1120-1229 MPa; Tensile strength: 1589-1674 MPa; Elongation (A50): 6.0%-8.0% 422-524 6% door anti-collision bar Yield strength: 1068-1164 ppm; Tensile strength: 1584-1599 ppm; Elongation (A50): 6.5%-8.0% 432-514 2.5% Top frame reinforcement plate Yield strength: 1012-1153 MPa; Tensile strength: 1450-1490 MPa; Elongation (A50): 6.0%-7.0% 442-504 3.5% B-pillar reinforcement plate Yield strength: 1103-1127 MPa; Tensile strength: 1493-1527 MPa; Elongation (A50): 6.0%-6.5% 412-501 5.0% Longitudinal beam connecting plate Yield strength: 1111-1236 MPa; Tensile strength: 1517-1580 MPa; Elongation (A50): 6.5%-8.5% 412-521 6.0% Longitudinal beam cover Yield strength: 992-1120 MPa; Tensile strength: 1450-1490 MPa; Elongation (A50): 5.5%-6.5% 442-511 2.3%

[0074] IV. Surface Quality of Quenching

[0075] This invention improves the surface quality of quenched steel coils by increasing the content of antioxidant elements such as Cr and Al and by controlling the rolling temperature. The changes in composition and rolling process are shown in Table 6.

[0076] Table 6 Implementation details of different processes

[0077] Example Al content Cr content Winding temperature / ℃ Surface iron oxide scale peeling 1 0.045% 0.25% 650 slight peeling 2 0.024% 0.15% 650 Severe shedding 3 0.024% 0.15% 550 Severe shedding 4 0.045% 0.25% 550 No shedding

[0078] Different embodiments were hot-stamped for trial production, and the surface condition after trial production is as follows: Figure 5 As shown, the surface oxide scale condition is related not only to the Al and Cr element content but also to the winding temperature. It can be seen that increasing the Al and Cr element content significantly improves the surface quality, and decreasing the winding temperature also significantly improves the surface quality. This is related to the grain boundary oxidation condition of the raw material surface. Related verification... Figure 6 As shown.

[0079] The surface conditions at different winding temperatures are shown below. When the winding temperature is 650℃, there is more severe grain boundary oxidation on the surface of the raw material. The grain boundary oxidation on the surface of the raw material provides more reaction sites and diffusion channels, which will aggravate and accelerate the surface oxidation during the heating process, thereby increasing the thickness of the iron oxide scale on the surface of the part. It can be seen that when the winding temperature is 550℃, the thickness of the iron oxide scale on the surface of the part is 7.96μm, and when the winding temperature is 650℃, the thickness of the iron oxide scale on the surface of the part is 10.65μm.

[0080] Based on 22MnB5, by adjusting the content of elements such as Mn, Cr, and Al, and the rolling process, the mechanical properties after hot pressing are improved, and the surface quality is also enhanced. After batch trial molding verification, this invention can replace cold-rolled products of the same specifications used in the market, and the surface quality of hot-pressed products is superior to that of cold-rolled products. Currently, all automotive hot-stamping steels used in the market are cold-rolled products. The ESP product achieves hot-pressing instead of cold-pressing, leveraging its cost advantage to widely replace existing market products. The current market size for automotive hot-stamping steel is approximately 1 million tons. If widely promoted, it can create considerable economic benefits. Based on a final market share of 40% and a profit of 300 yuan per ton of steel, the market potential could reach over 120 million yuan.

[0081] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0082] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line, characterized in that, Includes the following steps: The chemical composition of S1 hot-formed steel by mass percentage is as follows: C: 0.21-0.26%, Mn: 1.20-1.80%, Si: 0.20-0.35%, S≤0.003%, P≤0.015%, Cr: 0.20-0.47%, Alt: 0.04-0.07%, N≤0.005%, Ti: 0.045-0.055%, B: 0.0025-0.0035%, with the remainder being iron and unavoidable impurities; S2. The steel billet is continuously cast, and the continuous casting process parameters are set. S3. After being formed in the tundish and crystallizer, the continuously cast molten steel enters the induction heating furnace. S4. The steel billet enters the roughing mill of the ESP production line for rolling, and then enters the finishing mill for rolling into ESP hot-formed steel. The application of hot pressing and electric welding processes for S5 and ESP hot-formed steels and the setting of parameters.

2. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, The continuous casting process parameters in step S2 are set as follows: superheat 15-25℃, N content increase during casting ≤3ppm, AlS loss during casting ≤40ppm, target casting speed control is 4.5m / min during mixed casting and 5.0m / min after mixed casting, standard deviation of liquid level fluctuation in the crystallizer ≤1.5mm, and temperature difference of billet cross section ≤30℃.

3. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 2, characterized in that, In step S3, the molten steel for continuous casting is baked in the tundish for ≥2.0h at a temperature ≥1200℃, the immersion nozzle in the tundish is baked for ≥1.5h at a temperature ≥1200℃, and the crystallizer taper is 1.14-1.26%.

4. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, The process control requirements for the induction heating furnace in step S3 are an outlet temperature of 1150-1170℃ and a heating temperature controlled at the upper limit of 1250℃.

5. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, In step S4, the roughing mill of the ESP production line adopts R1 mill and R3 mill. The inlet temperature difference of R1 mill is ≤50℃, and the outlet temperature of R3 mill is >1000℃.

6. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, In step S4, the ESP production line sets the yield strength, tensile strength, and elongation of the billet raw material according to the different thickness specifications of the hot-formed steel. The thickness specification of the hot-formed steel is 1.2-1.6mm, the yield strength of the billet raw material is 441-467MPa, the tensile strength of the billet raw material is 615-632MPa, and the elongation of the billet raw material is 20.0-22.0%.

7. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, In step S4, the final rolling temperature of the finishing mill is controlled at 830±20℃, and the coiling temperature is controlled at 550±20℃.

8. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, The hot pressing process of the ESP hot-formed steel in step S5 uses a heating temperature of 900-950℃ and a heating time of the thickness of the hot-formed steel * 100 + 120s.

9. The method for producing hot-rolled hot-formed steel based on a fully headless continuous casting and rolling production line according to claim 1, characterized in that, The spot welding process of the ESP hot-formed steel in step S5 requires a welding current range of ≥1.0kA under specified welding time conditions.

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