600 MPa Grade High Formability Green Clean Surface Automotive Beam Steel Sheet and Its Manufacturing Method

Abstract

This invention provides a 600 MPa grade high-formability, green, clean surface automotive beam steel sheet and its manufacturing method. The chemical composition of the steel sheet, by mass percentage, is: C 0.07~0.09%, Si 0.05~0.12%, Mn 1.60~1.75%, Als 0.020~0.045%, Ti 0.050~0.070%, N≤0.008%, P≤0.018%, S≤0.008%, with Ti / C≤1.0, Ti×C≥0.0035, and the balance being Fe and unavoidable impurities. The manufacturing method includes smelting, continuous casting of medium-thin slabs, heating, two-stage controlled rolling and cooling, and EPS green, clean surface treatment. The steel plate has a yield strength ReL≥550 MPa, tensile strength Rm≥600 MPa, elongation after fracture A≥22%, strength-ductility product ≥13200 MPa·%, yield strength ratio ≤0.90, work hardening index n≥0.18, surface roughness Ra=3.0~4.5 μm, surface cleanliness Sa not lower than level 3, and finished product thickness 3~12 mm.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically relating to a 600 MPa grade high formability green clean surface automotive beam steel plate and its manufacturing method. Background Technology

[0002] In recent years, with the rapid development of the automotive industry, automakers have increasingly higher requirements for the surface quality, environmental performance, and production costs of steel sheets. As a crucial component of the automotive structure, the lightweight manufacturing and green production processes of automotive beams have become a focus of industry attention. For 600 MPa grade automotive beam steel, not only is high strength required, but also good formability to adapt to complex stamping and roll forming processes.

[0003] While traditional 600 MPa grade automotive beam steel plates can achieve strength targets by adding elements such as Nb and V, they often face problems such as low elongation and insufficient formability. Other technologies use offline heat treatment to control the microstructure, but this increases the number of processes, energy consumption, and costs.

[0004] The published patent CN107649522A proposes a method for controlling the surface quality of hot-rolled steel coils, but the steel plates produced by it have a large surface roughness, which is not conducive to subsequent processes.

[0005] The method proposed in the published patent CN101906584A requires the addition of Cr element, which is costly.

[0006] The published patent CN103320683A relies on the pickling process, which has poor environmental performance.

[0007] The steel plate disclosed in patent CN105369134A has a relatively low strength grade.

[0008] Therefore, how to stably obtain 600 MPa grade automotive beam steel sheets with excellent comprehensive performance on a hot rolling line without relying on offline heat treatment and the addition of precious metal elements, especially to achieve a combination of high strength, high elongation (high strength-ductility product) and excellent surface quality, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a 600 MPa grade high-formability, green, clean surface automotive beam steel sheet and its manufacturing method. Based on the C-Mn series steel composition, this invention employs a low-Si, Ti micro-alloying composition system without the addition of expensive alloying elements. Through an innovative two-stage controlled cooling process combined with EPS green, clean surface treatment, a steel sheet for automotive beams that combines high strength and high formability is obtained.

[0010] The first technical solution of this invention is to provide a 600 MPa grade high formability green clean surface automotive beam steel plate, the specific technical solution of which is as follows: the chemical composition by mass percentage is C: 0.07%~0.09%, Si: 0.05%~0.12%, Mn: 1.60%~1.75%, Als: 0.020%~0.045%, Ti: 0.050%~0.070%, N≤0.008%, P≤0.018%, S≤0.008%, with the balance being Fe and unavoidable impurities.

[0011] The rationale for the alloy design of this invention is as follows:

[0012] Carbon (C): Carbon is the core element for ensuring strength. If the C content is too low, it is difficult to achieve a strength of 600 MPa; if the content is too high, it will worsen the weldability of the steel plate and reduce its elongation. Taking all factors into consideration, the C content is limited to 0.07%~0.09%.

[0013] Si: Si plays a role in solid solution strengthening and deoxidation. To prevent the formation of complex iron oxide scale that is difficult to remove during hot rolling, which would affect the subsequent EPS treatment effect, the present invention limits the Si content to 0.05%~0.12%.

[0014] Mn: Mn is a key element for improving the hardenability of steel plates. It can reduce the critical cooling rate for bainite transformation and expand the process window. To obtain the target duplex microstructure, the Mn content needs to be increased to 1.60%~1.75%. This range can provide sufficient hardenability for the steel plate while avoiding center segregation of the billet.

[0015] Ti: Ti is the core strengthening element in this invention. The Ti content is significantly increased to 0.050%~0.070%, which serves two purposes: first, it precipitates TiN particles at high temperatures, pinning grain boundaries and refining austenite grains; second, it precipitates nano-sized TiC particles during cooling, producing a strong precipitation strengthening effect. If the Ti content is below 0.050%, the strengthening effect is insufficient; if it is above 0.070%, large-sized TiN inclusions are easily formed. Therefore, this invention limits the Ti content to 0.050%~0.070%.

[0016] Als: Al is a deoxidizing element in steel, which reduces oxide inclusions and purifies the steel, thus improving the formability of the steel sheet. Simultaneously, Al can form AlN precipitates, refining the grain size. Therefore, the optimal range of Als in this invention is between 0.020% and 0.045%.

[0017] Nitrogen (N) is a key element in the formation of Ti(C,N) composite precipitates. Ti has a strong affinity for N, and an appropriate amount of N can promote the formation of dispersed TiN particles at high temperatures, thereby enhancing the pinning of austenite grain boundaries and refining the grains. Simultaneously, N can also participate in the precipitation of Ti(C,N) at low temperatures, moderately increasing the strengthening increment. However, excessively high N content (>0.008%) will lead to excessive precipitation and coarsening of TiN at high temperatures, which will not only weaken the fixation of C by Ti and the subsequent TiC strengthening effect, but also form large cubic inclusions, severely impairing the toughness and fatigue properties of the steel. Therefore, this invention limits N to ≤0.008%.

[0018] S and P: Both S and P are harmful elements. The lower their content, the better. If the content is too low, the production cost will be high. Under the premise of not affecting the performance of the steel plate, their content is limited to P≤0.018% and S≤0.008%.

[0019] Ti-C synergistic relationship: To maximize the precipitation strengthening effect of Ti, the atomic equivalent ratio of Ti to C needs to be controlled. Ti / C ≤ 1.0 can prevent the formation of coarse cementite and ensure that C preferentially combines with Ti to form nanoscale TiC; Ti×C ≥ 0.0035 ensures a sufficient volume fraction of precipitates to ensure the strengthening effect.

[0020] Preferably, the microstructure of the steel plate comprises ferrite and bainite, wherein the volume fraction of ferrite is 50%~60%, the volume fraction of bainite is 40%~50%, and the average grain size is 10~12. This dual-phase microstructure, with the soft ferrite phase ensuring high plasticity and the hard bainite phase ensuring high strength, and the slight hardness difference between the soft and hard phases endowing the steel plate with excellent hole-expanding performance and fatigue resistance.

[0021] Preferably, the steel plate has a yield strength ReL≥550 MPa, tensile strength Rm≥600 MPa, elongation after fracture A≥22%, strength-ductility product ≥13200 MPa·%, yield strength ratio ≤0.90, work hardening index n≥0.18, surface roughness Ra=3.0~4.5 μm, surface cleanliness Sa not lower than level 3, and finished product thickness of 3~12 mm.

[0022] The second technical solution of this invention provides a method for manufacturing 600 MPa grade high formability green and clean surface automotive beam steel plates, including smelting, continuous casting of medium and thin slabs, heating, two-stage controlled rolling and cooling process, and EPS green and clean surface treatment process. The specific steps of this manufacturing method are as follows:

[0023] Medium-thin slab continuous casting: The medium-thin slab continuous casting process is adopted, and the thickness of the continuously cast slab is controlled at 135~200 mm. Compared with the traditional 225~250 mm thick slab continuous casting and rolling process, medium-thin slab continuous casting can effectively shorten the production line length and reduce energy consumption.

[0024] Heating process: The continuously cast billet is heated in a walking beam furnace at a temperature controlled at 1220~1260 ℃ for 1~3 h. Increasing the heating temperature to 1220~1260 ℃ aims to fully dissolve the relatively high Ti content (0.050%~0.070%), thus preparing for the subsequent dispersion precipitation of nano-sized TiC.

[0025] Two-stage controlled rolling and cooling process: This is the core process for obtaining a ferrite + bainite dual-phase microstructure.

[0026] Controlled rolling stage: A two-stage controlled rolling process is adopted, with a finishing rolling start temperature of 980~1030 ℃ and a finishing rolling temperature of 820~860 ℃. The finishing rolling temperature of 820~860 ℃ is about 90~130 ℃ higher than the Ar3 temperature, ensuring that the rolling is carried out in the non-recrystallization region of austenite, and the accumulated deformation can effectively refine the austenite grains before phase transformation.

[0027] The key two-stage cooling process: immediately after final rolling, the machine enters a finely designed cooling path.

[0028] The first stage is the rapid cooling stage: rapid water cooling to 600~650℃ at a cooling rate of ≥25 ℃ / s. The main purpose of this stage is to allow the supercooled austenite to quickly pass through the high-temperature region, effectively suppressing the excessive growth of polygonal ferrite and the formation of pearlite. At the relaxation endpoint of 600~650 ℃, about 50%~60% of the supercooled austenite transforms into fine equiaxed ferrite, while the remaining carbon-rich austenite is uniformly distributed in the ferrite matrix.

[0029] The second stage is the slow cooling phase transformation stage: Subsequently, weak water cooling is performed at a cooling rate of ≤10℃ / s to slowly cool the strip to a coiling temperature of 520~580℃. During this slow cooling process, the carbon-rich supercooled austenite gradually transforms into fine bainite within the range between the relaxation end temperature and the coiling temperature, ultimately forming a ferrite + bainite dual-phase structure. The coiling temperature window of 520~580℃ provides the optimal driving force for the nanoscale precipitation of TiC.

[0030] EPS green cleaning surface treatment process: After the steel coil is uncoiled and straightened, it enters the EPS (Eco Pickled Surface) unit. Using water and steel grit as the medium, hot-rolled iron oxide scale is removed by physical jetting. The core component of the EPS unit is two sets of high-speed rotating turbines positioned vertically opposite each other. The turbine blades accelerate the mixture of steel grit and water to a linear velocity of 65~75 m / s, and simultaneously spray it onto the steel plate surface from both sides at a 60° incident angle, peeling off the iron oxide scale through abrasive impact.

[0031] Preferably, in the EPS process window, the turbine speed is 1800~2100 rpm, the steel plate travel speed is 30~45 m / min, the steel grit mesh is 45~55 mesh, and the corresponding average particle size is about 270~350 μm.

[0032] Furthermore, to ensure a surface roughness Ra ≤ 5.0 μm and a cleanliness Sa ≥ 3, the process parameters and steel plate properties must satisfy the following relationship:

[0033]

[0034] In the formula: n Turbine speed, rpm; v Let be the speed of the steel plate, in m·min -1 ; d The average particle size of the steel shot is in μm. s is the tensile strength of the steel plate, in MPa; K is the equipment constant, ranging from 0.10 to 0.18.

[0035] Preferred, When the range is between 20 and 30, steel plates with Ra = 3.0 to 4.5 μm can be stably obtained, and the cleanliness Sa reaches level 3.

[0036] After EPS treatment, the steel plate surface exhibits a uniform silver-gray metallic luster, with no oxide scale residue or oil stains. An environmentally friendly corrosion inhibitor is then used to form a protective film on the steel plate surface after treatment.

[0037] This invention provides a 600 MPa grade high formability green clean surface automotive beam steel plate and its manufacturing method. Compared with the prior art, the beneficial effects are as follows:

[0038] 1. Excellent overall performance: Through the synergistic effect of innovative composition and two-stage controlled cooling process, ferrite + bainite dual-phase structure is directly obtained on the hot rolling line, achieving a perfect combination of high strength (Rm≥600 MPa) and high formability (A≥22%, work hardening index n≥0.18), with a strength-ductility product (Rm×A)≥13200 MPa·%, which is superior to traditional products of the same level.

[0039] 2. Simple and efficient process: The entire process is completed in one go on the hot rolling line, without any subsequent offline heat treatment, which greatly shortens the production cycle and reduces energy consumption and production costs.

[0040] 3. Green and environmentally friendly: The pickling process is completely eliminated. EPS green cleaning surface treatment is adopted, and iron oxide scale is removed with water and steel sand as the medium. Wastewater and waste sand are recycled throughout the process. There is no acid mist or waste liquid discharge, realizing truly green and clean production.

[0041] 4. Controllable surface quality: After EPS treatment, the steel plate surface presents a uniform and clean surface with a surface roughness Ra=3.0~4.5 μm, providing an excellent surface foundation for subsequent coating and stamping. Detailed Implementation

[0043] The present invention will be further illustrated below with reference to embodiments. These embodiments are merely descriptions of the best mode of implementation and do not limit the scope of the invention in any way.

[0044] This invention is achieved through the following process steps: A medium-thin slab continuous casting process is adopted, with the slab thickness controlled at 135~200 mm; the slab is heated in a furnace to 1220~1260 ℃ and held for 1~3 h to ensure sufficient solid solution of alloying elements; a two-stage controlled rolling process is adopted, with a finishing rolling start temperature of 980~1030 ℃ and a finishing rolling temperature of 820~860 ℃; after finishing rolling, an innovative two-stage controlled cooling process is adopted: the first stage uses rapid water cooling at a cooling rate of ≥25 ℃ / s to 600~650 ℃; the second stage uses air cooling or weak water cooling at a cooling rate of ≤10 ℃ / s to a coiling temperature of 520~580 ℃; the finished product thickness is 3~12 mm.

[0045] Before EPS green cleaning surface treatment, the steel plate is straightened. The steel coil enters the EPS unit, and water and steel grit are used as the medium to remove the iron oxide scale on the surface of the steel plate by high-speed rotating turbine. The turbine speed is set to 1800~2100 rpm, and the steel plate travel speed is adjusted to 30~45 m / min. After treatment, an environmentally friendly corrosion inhibitor is used to form a protective film on the surface of the steel plate.

[0046] The following embodiments are merely some preferred implementations of the present invention and do not limit the scope and technical means of the invention in any way. The chemical composition of the embodiments of the present invention is shown in Table 1; the process parameters for continuous casting, heating, two-stage controlled rolling and cooling, and EPS of the steel in the embodiments of the present invention are shown in Table 2; the microstructure parameters of the steel in the embodiments of the present invention are shown in Table 3; and the mechanical property parameters and surface quality of the steel in the embodiments of the present invention are shown in Table 4.

[0047] Table 1 Chemical composition (wt, %) of embodiments of the present invention 1 0.085 0.12 1.60 0.013 0.002 0.004 0.036 0.052 44.2 0.61 2 0.070 0.05 1.68 0.018 0.003 0.005 0.044 0.055 38.5 0.79 3 0.075 0.11 1.75 0.011 0.005 0.006 0.038 0.050 37.5 0.67 4 0.071 0.07 1.65 0.010 0.001 0.003 0.027 0.063 44.7 0.89 5 0.073 0.08 1.72 0.012 0.008 0.004 0.025 0.069 50.4 0.95 6 0.080 0.10 1.62 0.015 0.004 0.006 0.020 0.058 46.4 0.73 7 0.088 0.05 1.70 0.014 0.006 0.004 0.045 0.070 61.6 0.80 8 0.072 0.06 1.63 0.009 0.002 0.005 0.032 0.054 38.9 0.75 Table 2. Process parameters for continuous casting, heating, two-stage controlled rolling and cooling processes, and EPS in embodiments of the present invention. 1 135 1220 1.8 1010 820 28 620 8 545 1900 38 50.0 0.49 24.5 0.14 2 170 1250 1.5 1020 830 30 610 9 530 1950 42 46.4 0.50 23.2 0.16 3 200 1245 2.0 985 845 26 630 7 540 1850 36 51.4 0.49 25.2 0.16 4 135 1260 1.2 995 835 27 615 8 525 2000 44 45.5 0.49 22.3 0.16 5 170 1240 1.6 1015 840 29 625 8 535 2050 40 51.3 0.48 24.6 0.15 6 135 1225 1.0 980 825 30 600 6 550 1800 35 51.4 0.51 26.2 0.12 7 200 1260 2.0 1030 850 32 640 10 540 2100 45 46.7 0.47 22.0 0.18 8 170 1230 1.4 1000 830 26 615 8 530 1880 39 48.2 0.50 24.1 0.15 (Note: d The average particle size of steel shot is approximately 310 μm for 45-55 mesh. sTake the measured tensile strength Rm, in MPa; in the table d / s During calculation d In μm, s (Measured in MPa.) Table 3. Microstructure parameters of steel in the embodiments of the present invention 1 53 47 11 2 53 47 11 3 56 44 11 4 51 49 11 5 50 50 12 6 58 42 10 7 52 48 11 8 55 45 11 (Note: The volume fractions of ferrite and bainite are obtained from EBSD statistics, and their sum is approximately 100%. The influence of trace amounts of retained austenite or carbides (volume fraction <1%) on performance is negligible within the statistical error range, so they are not listed separately.) Table 4 Mechanical property parameters and surface quality of the steel in the embodiments of the present invention 1 9.0 560 666 24.0 15984 0.84 3.5 0.22 3 2 6.0 595 663 23.5 15581 0.90 3.8 0.24 3 3 5.5 597 671 24.0 16104 0.89 4.0 0.26 3 4 4.0 595 662 23.5 15557 0.90 3.5 0.21 3 5 3.0 610 680 25.5 17340 0.90 3.6 0.19 3 6 12.0 558 648 24.5 15876 0.86 3.2 0.20 3 7 3.5 610 690 22.0 15180 0.88 4.0 0.22 3 8 7.0 575 655 24.0 15720 0.88 3.6 0.18 3 As can be seen from Table 4, the tensile strength Rm of the steel in the embodiments of the present invention is 648~690 MPa, all reaching the design strength of 600 MPa or more; the yield strength ReL is 558~610 MPa; the elongation after fracture A is ≥22%; the strength-ductility product is ≥13200 MPa·%, the yield strength ratio is ≤0.90; the work hardening index n is ≥0.18; the surface roughness Ra is 3.0~4.5 μm; and the surface cleanliness Sa is not lower than level 3.

[0048] It is hereby noted that the above embodiments are only for illustrating the technical concept and features of the present invention, and are not intended to limit the present invention. Any equivalent substitutions or modifications made without departing from the essence of the present invention fall within the protection scope of the present invention.

Claims

1. A 600 MPa grade high formability green clean surface automotive beam steel plate, characterized in that, By weight percentage, it comprises the following components: C: 0.07%~0.09%, Si: 0.05%~0.12%, Mn: 1.60%~1.75%, Als: 0.020%~0.045%, Ti: 0.050%~0.070%, N≤0.008%, P≤0.018%, S≤0.008%, and Ti / C≤1.0, Ti×C≥0.0035, with the balance being Fe and unavoidable impurities.

2. The 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 1, characterized in that, The microstructure of the steel plate includes ferrite and bainite, wherein the volume fraction of ferrite is 50%~60%, the volume fraction of bainite is 40%~50%, and the average grain size is 10~12.

3. The 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 1, characterized in that, The steel plate has a yield strength ReL≥550 MPa, tensile strength Rm≥600 MPa, elongation after fracture A≥22%, strength-ductility product ≥13200 MPa·%, yield strength ratio ≤0.90, work hardening index n≥0.18, surface roughness Ra=3.0~4.5 μm, surface cleanliness Sa not lower than level 3, and finished product thickness of 3~12 mm.

4. A method for manufacturing 600 MPa grade high formability green and clean surface automotive beam steel plate according to any one of claims 1 to 3, comprising smelting, continuous casting of medium and thin slabs, heating, two-stage controlled rolling and cooling process, and EPS green and clean surface treatment process, characterized in that: Two-stage controlled rolling and cooling process: Two-stage controlled rolling is adopted, with a finishing rolling start temperature of 980~1030 ℃ and a finishing rolling temperature of 820~860 ℃; After final rolling, a two-stage controlled cooling process is adopted: in the first stage, the strip is water-cooled to 600~650℃ at a cooling rate of ≥25 ℃ / s; in the second stage, the strip is water-cooled at a cooling rate of ≤10 ℃ / s to cool it to the coiling temperature of 520~580℃. EPS green cleaning surface treatment process: After the steel coil is uncoiled and straightened, it enters the EPS unit. Using water and steel grit as the medium, a steel grit-water mixed slurry is sprayed through a high-speed rotating turbine to remove the iron oxide scale on the surface of the steel plate by physical impact.

5. The method for manufacturing a 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 4, characterized in that, The process of continuous casting of medium and thin slabs is adopted, and the thickness of the continuously cast slab is 135~200 mm.

6. The method for manufacturing a 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 4, characterized in that, The continuously cast billet is heated in a walking beam furnace at a temperature of 1220~1260 ℃ and held for 1~3 h.

7. The method for manufacturing a 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 4, characterized in that, The steel grit has a mesh size of 45-55.

8. The method for manufacturing a 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 4, characterized in that, In the EPS green cleaning surface treatment process, the turbine speed is 1800~2100 rpm and the steel plate travel speed is 30~45 m / min.

9. The method for manufacturing a 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 4, characterized in that, The relationship between process parameters and steel plate properties is as follows: In the formula: n Turbine speed, rpm; v Let be the speed of the steel plate, in m·min -1 ; d The average particle size of the steel shot is in μm. σ is the tensile strength of the steel plate, in MPa; K is the equipment constant, ranging from 0.10 to 0.

18.

10. The method for manufacturing a 600 MPa grade high formability green clean surface automotive beam steel plate according to claim 9, characterized in that, The range is between 20 and 30.