CN117187682B - Steel for 1200MPa battery pack for new energy vehicles and preparation method thereof - Google Patents

Abstract

The present invention provides a steel for a 1200MPa battery pack for a new energy vehicle and a preparation method thereof. The components of the steel are as follows by weight percentage: C: 0.20% to 0.24%, Si: 0.20% to 0.40%, Al: 0.40% to 0.60%, Mn: 0.9% to 1.1%, Cr: 0.30% to 0.60%, Mo: 0.20% to 0.40%, B: 0.004% to 0.006%, P≤0.015%, S≤0.003% , Ti: 0.015-0.035%, the balance is Fe and inevitable impurities; the preparation method includes smelting, continuous casting, hot rolling, pickling, cold rolling, continuous galvanizing, and skin pass; the battery pack steel produced by the present invention has a tensile strength of more than 1200MPa, a yield strength of 900-1050MPa, an elongation of ≥12%, a hole expansion rate of ≥55%, an isothermal yield decay of less than 200MPa at 400°C for 5 minutes, and an isothermal yield decay of less than 500MPa at 800°C for 5 minutes.

Description

Steel for 1200MPa battery pack for new energy vehicles and preparation method thereof

Technical Field

The present invention belongs to the field of metal materials, and in particular relates to a steel for a 1200MPa battery pack for a new energy vehicle and a preparation method thereof.

Background technique

The battery pack consists of an upper cover, a crossbeam, a frame, and a lower plate. The crossbeam and frame are used to ensure the safety of the battery pack, the strength of the module installation, and to effectively resist side impacts and ensure the safety of the modules inside the battery. Therefore, the crossbeam and frame require high-strength steel or ultra-high-strength steel as materials. At present, the steel used for the crossbeam and frame is mainly based on traditional high-strength steel, such as high-strength dual-phase steel, complex phase steel, martensitic steel, quenching and partitioning steel, etc.

The Chinese patent application number 2018115042560 discloses a high-N content ultrafine-grained 1200MPa cold-rolled dual-phase steel and its production process, C: 0.14% to 0.17%, Si: 0.2% to 0.3%, Mn: 1.5% to 2.0%, P≤0.015%, S≤0.010%, V: 0.10% to 0.15%, Cr: 0.03% to 0.04%, Als: 0.02 to 0.03%, Ti: 0.03 to 0.06%, N: 0.012% to 0.018%, and the rest is Fe and unavoidable impurities. A steel plate with a tensile strength of 1200MPa is obtained, but a high content of N is added to the steel, which greatly increases the smelting cost. In terms of performance, the yield strength is too low to bear the application conditions of steel for battery packs such as beams and frames.

Publication No. CN109280857A discloses a 1200MPa grade ultra-fast cold-rolled dual-phase steel plate and its preparation method. The main chemical composition of the disclosed dual-phase steel is: C: 0.12% to 0.17%, Si: 0.3% to 0.6%, Mn: 2.0% to 2.4%, P≤0.015%, S≤0.008%, Als: 0.03 to 0.06%, Ti: 0.03 to 0.06%, N≤0.005%, and the rest is Fe and unavoidable impurities. The steel has the following properties: yield strength 820 to 950MPa, tensile strength: 1200 to 1350MPa, and elongation 5 to 10%. The steel preparation method involves ultra-fast cooling of 130 to 150℃/s, which is difficult to achieve in actual production processes, and the product drawing performance is difficult to meet the needs of more complex parts forming.

Summary of the invention

The object of the present invention is to overcome the above-mentioned problems and shortcomings and to provide a 1200MPa battery pack steel for new energy vehicles having high temperature tensile properties and excellent resistance to steel plate softening under simulated fire conditions and a preparation method thereof.

The object of the invention is achieved in this way:

A 1200MPa battery pack steel for new energy vehicles, the composition of the steel is as follows by weight percentage: C: 0.20% to 0.24%, Si: 0.20% to 0.40%, Al: 0.40% to 0.60%, Mn: 0.9% to 1.1%, Cr: 0.30% to 0.60%, Mo: 0.20% to 0.40%, B: 0.004% to 0.006%, P≤0.015%, S≤0.003%, Ti: 0.015 to 0.035%, the balance is Fe and unavoidable impurities. Preferably, in the 1200MPa battery pack steel, 0.36≤C+Mn/6≤0.4.

The microstructure of the steel for the battery pack includes critical zone ferrite, epitaxial ferrite, tempered martensite, carbide and other small amounts of unidentified phases, wherein each microstructure is calculated by volume as follows: critical zone ferrite 8% to 12%, epitaxial ferrite 10% to 15%, tempered martensite 70% to 75%, and carbide 2.2% to 3.5%; preferably, the carbide is mainly θ-type carbide, the volume fraction of θ-type carbide accounts for more than 70% of all carbides, and the average grain size is 45 to 80 nm.

The steel for the battery pack has a tensile strength of more than 1200 MPa, a yield strength of 900-1050 MPa, an elongation of ≥12%, a hole expansion rate of ≥55%, an isothermal yield decay of less than 200 MPa at 400°C for 5 minutes, and an isothermal yield decay of less than 500 MPa at 800°C for 5 minutes.

The reasons for the composition design of the present invention are as follows:

C: C is a necessary element in high-strength steel to ensure the strength of the steel plate. In the present invention, the C addition range is strictly controlled, and the addition of C ensures the C behavior in the tempered martensite, the content of θ-type carbides and the grain size.

Si: Si is a conventional solid solution strengthening element, which plays a role in strengthening the matrix. In the present invention, the main role of Si is to inhibit the precipitation of carbides in tempered martensite to a certain extent and control the content of carbides between tempered martensite. The invention adopts a low Si design because it effectively controls the surface quality of cold-rolled steel sheets. High Si addition will significantly affect the surface of the steel sheet, forming internal oxidation and grain boundary oxidation layers; in addition, excessive Si addition also seriously affects the surface quality of galvanized steel sheets, forming a "leak plating" phenomenon.

Mn: Mn is a conventional strengthening element in steel, and the composition system of high-strength steel is mainly based on the C-Mn system. The characteristic of the present invention is that a low Mn design is adopted, because the conventional addition of 2.0% or even more Mn content above the 1000MPa level significantly improves the hardenability of the steel plate, resulting in the formation of bainite and martensite in the hot coiling stage, which seriously increases the difficulty of cold rolling; furthermore, the composite addition of C and Mn is prone to cause C/Mn segregation, resulting in excessive edge strength, which also increases the difficulty of cold rolling. Therefore, a low Mn composition design is adopted to control the Mn content to 0.9-1.1%, 0.36≤C+Mn/6≤0.4, which can ensure the strength of the steel plate and have good welding performance.

Al: Al is conventionally used as a deoxidizer in steel to regulate the O content in steel. In the present invention, the role of Al is crucial. First, Al is added to replace part of the role of Si, taking into account the surface quality while controlling the carbide precipitation content and carbide state. Second, Al promotes pearlite spheroidization in the hot coiling stage, inhibits bainite and martensite phase transformations that may be caused by high hardenability, ensures a lower hot rolling reduction, and better smoothness to the cold rolling stage. Furthermore, Al addition effectively regulates the austenite range, ensuring the strength-plastic matching performance of the steel plate.

Cr: Cr is a conventional added element, which plays a role in solid solution strengthening and improving hardenability similar to Mn. In the present invention, Cr plays a crucial role in that it replaces Mn and is added to improve hot-rolled edge cracks.

Mo: Mo is a conventional added element, which plays a role in solid solution strengthening and improving hardenability similar to Mn. In the present invention, Mo plays a crucial role in replacing Mn to improve hot-rolled edge cracks and enhance hot-rolled surface quality.

B: B is a crucial element of the present invention. The main function of B is to improve the hardenability of the steel plate and prevent the formation of epitaxial hot bodies in the slow cooling and fast cooling stages.

Ti: Conventional addition of Ti can capture free N atoms in steel and play a role in fixing N. At the same time, TiN can precipitate during the solidification process and play a role in pinning grain boundaries. Ti(C,N) precipitates during the hot rolling stage and plays a role in pinning the original austenite grain boundaries and refining the original austenite grains. In the present invention, the role of Ti is more important in that it exists in the form of precipitation between tempered martensite, captures H atoms, and resists hydrogen-induced cracking.

P: P is a harmful element in steel, and the lower its content, the better. Considering the cost, the content of P is controlled within P≤0.015% in the present invention.

S: S is a harmful element in steel, and the lower its content, the better. Considering the cost, the content of S in the present invention is controlled to S≤0.003%.

The second technical solution of the present invention is to provide a method for preparing 1200MPa battery pack steel for new energy vehicles, including smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing, and skin pass;

Continuous Casting:

Through smelting in a converter, the alloy composition within the above range is obtained, the middle package temperature is 1530-1560℃, the billet drawing speed is 0.6-0.8m/min, and the drawing speed is controlled to prevent the "steel leakage alarm" caused by excessive drawing speed.

Hot Rolled:

The heating temperature is between 1260 and 1310 ° C, and the isothermal temperature is 120 to 160 minutes to ensure the precipitation of Ti atoms, which has a good effect on the solidification of N on the steel plate, and ensures the precipitation of Ti (C, N), which plays a role in pinning the original austenite grain boundary and refining the original austenite grain; the control time range is to prevent the precipitation of TiN from coarsening and causing grain boundary embrittlement on the basis of ensuring sufficient solid solution of elements. The starting rolling temperature is between 1080 and 1130 ° C, and the final rolling temperature is above 900 to 950 ° C. The starting rolling temperature ensures the recrystallization behavior of austenite; the final rolling temperature prevents the formation of pro-eutectoid ferrite. After that, ultra-fast cooling + laminar cooling is carried out, with an ultra-fast cooling rate of 5 to 8 ° C / s and a laminar cooling rate of 2-3 ° C / s. Ultra-fast cooling prevents the supercooled austenite from quickly transitioning to the low temperature range and prevents excessive element distribution; laminar cold precipitation must be pro-eutectoid ferrite to promote pearlite spheroidization. The coiling temperature is 450-490°C, which inhibits oxidation and the formation of grain boundary oxide layer during this stage and ensures the surface quality of the steel plate.

Pickling:

Remove the iron oxide scale generated on the hot-rolled surface, and the pickling speed is 90-110m/min. Prevent the pickling from being too slow to increase production costs, and prevent the speed from being too fast to ensure the surface quality of the cold-rolled steel plate.

Cold Rolling:

The cold rolling reduction rate is 40% to 58%, ensuring a cold rolling reduction of more than 40% to promote tissue fiberization in the cold rolling configuration; at the same time, prevent the cold rolling reduction rate from being too high, resulting in excessive deformation resistance and difficulty in rolling to the target thickness.

Continuous galvanizing:

① The heating temperature is between 920 and 950°C, and the isothermal time is between 10 and 40 seconds. The isothermal temperature ensures the austenitizing temperature and controls the ferrite content in the critical zone between 8% and 12%. The isothermal time is strictly controlled to prevent the austenite grains in the critical zone from growing, and to ensure that the austenite phase deformation nucleation is completed and equiaxed.

② The slow cooling temperature is 720-780°C, and the slow cooling rate is controlled at 1-3°C/s; the epitaxial ferrite content is controlled at 10%-15%.

③ Rapidly cool to below 500℃ at a cooling rate greater than 45℃/s to play a pre-cooling role to prevent subsequent large undercooling from causing poor plate shape.

④ Quench directly in 80-95℃ water, then pickle to remove surface oxide scale.

⑤ Reheating: Heat to 450-470℃ at a heating rate of 10-20℃/s, with an isothermal time of 10-18s. Strictly control the temperature and time to prevent excessive carbide precipitation, and control the type and precipitation content of carbides.

⑥ Alloying hot-dip galvanizing: The steel strip is galvanized in a zinc pot and then enters an alloying furnace for isothermal treatment; the alloying temperature is 480-490°C and the isothermal treatment is 20-30s. The temperature is strictly controlled to prevent the temperature from being too low, which will lead to insufficient Fe content in the coating. The Zn-Fe layer is mainly composed of δ phase, and the temperature is too high, which will lead to an increase and coarsening of carbides. The beneficial effects of the present invention are:

(1) The steel plate of the present invention takes into account the composition control in the whole process, and effectively controls the production cost in multiple dimensions, such as composition, process and many other aspects.

(2) The present invention adopts the idea of coupling composition and process to innovatively propose a process design of quenching, water inlet and then heating, which ensures the excellent performance of the steel plate, with a tensile strength of more than 1200MPa, a yield strength of 900-1050MPa, an elongation of ≥12%, a hole expansion rate of ≥55%, an isothermal yield decay of less than 200MPa at 400°C for 5 minutes, and an isothermal yield decay of less than 500MPa at 800°C for 5 minutes.

(3) The battery pack steel of the present invention not only achieves static and dynamic performance, but also fully considers the problem of steel plate strength attenuation in combination with the design, thereby achieving the primary purpose of protecting the safety of the battery pack steel.

Detailed ways

The present invention will be further described below by way of examples.

The embodiment of the present invention carries out smelting, continuous casting, hot rolling, pickling, cold rolling, continuous galvanizing and skin-passing according to the component ratio of the technical solution; the characteristics are:

Hot Rolled:

Heating temperature 1260 ~ 1310 ℃, isothermal 120 ~ 160min; rolling temperature 1080 ~ 1130 ℃, final rolling temperature 900 ~ 950 ℃, followed by ultra-fast cooling + laminar cooling, ultra-fast cooling rate 5 ~ 8 ℃ / s, laminar cooling rate 2-3 ℃ / s, coiling temperature 450 ~ 490 ℃;

Pickling:

Pickling speed 90～110m/min;

Cold Rolling:

The cold rolling reduction rate is 40% to 58%;

Continuous galvanizing:

① The heating isothermal temperature is between 920 and 950°C, and the isothermal time is between 10 and 40 seconds.

② Slow cooling temperature 720 ~ 780 ℃, slow cooling rate controlled at 1 ~ 3 ℃ / s,

③ Rapidly cool to below 500℃ at a cooling rate greater than 45℃/s,

④ Directly quench in 80-95℃ water,

⑤ Reheating: Heat to 450-470℃ at a heating rate of 10-20℃/s, isothermal time is 10-18s,

⑥ Alloying hot-dip galvanizing: The steel strip is galvanized in a zinc pot and then enters an alloying furnace for isothermal treatment. The alloying temperature is 480-490°C and the isothermal treatment lasts for 20-30 seconds.

Furthermore, during the continuous casting process, the temperature of the tundish is 1530-1560°C, and the billet drawing speed is 0.6-0.8 m/min.

The composition of the steel of the present invention is shown in Table 1. The main process parameters of continuous casting and hot rolling of the steel of the present invention are shown in Table 2. The main process parameters of annealing of the steel of the present invention are shown in Table 3. The properties of the steel of the present invention are shown in Table 4. The microstructure (vol.%) of the steel of the present invention is shown in Table 5.

Table 1 Composition of steel according to the present invention (wt%)

Table 2 Main process parameters of continuous casting and hot rolling of steel in the embodiment of the present invention

Table 3 Main heat treatment process parameters for annealing of steel in the embodiment of the present invention

Table 4 Properties of steel in the examples of the present invention

Table 5 Microstructure of steel of the present invention (vol.%)

From the above, it can be seen that the microstructure of the steel for battery packs produced by the present invention includes critical zone ferrite, epitaxial ferrite, tempered martensite, carbide and other small amounts of unidentified phases, wherein each microstructure is calculated by volume as follows: critical zone ferrite 8% to 12%, epitaxial ferrite 10% to 15%, tempered martensite 70% to 75%, carbide 2.2% to 3.5%; preferably, the carbide is mainly θ-type carbide, the volume fraction of θ-type carbide accounts for more than 70% of all carbides, and the average grain size is 45 to 80nm; the tensile strength is above 1200MPa, the yield strength is 900 to 1050MPa, the elongation is ≥12%, the hole expansion rate is ≥55%, the isothermal 5min yield decay at 400°C is less than 200MPa, and the isothermal 5min yield decay at 800°C is less than 500MPa.

In order to describe the present invention, the present invention is appropriately and fully illustrated by the examples in the above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims (7)

1. The 1200MPa battery pack steel for the new energy automobile is characterized by comprising the following ：C：0.20%～0.24%,Si：0.20%～0.40%,Al：0.40%~0.60%,Mn：0.9%～1.1%,Cr：0.30%~0.60%,Mo：0.20%~0.40%,B：0.004%~0.006%,P≤0.015%,S≤0.003%,Ti：0.015～0.035%, parts by weight of Fe and unavoidable impurities as the balance.

2. The 1200MPa battery pack steel for a new energy automobile according to claim 1, wherein C+Mn/6 is more than or equal to 0.36 and less than or equal to 0.4.

3. The 1200MPa battery pack steel for new energy vehicles according to claim 1, wherein the battery pack steel microstructure comprises critical zone ferrite, epitaxial ferrite, tempered martensite, carbide and other minor unidentified phases, wherein each microstructure comprises, in volume percent: 8% -12% of ferrite in the critical area, 10% -15% of epitaxial ferrite, 70% -75% of tempered martensite and 2.2% -3.5% of carbide.

4. The 1200MPa battery pack steel for a new energy automobile according to claim 3, wherein the carbides are mainly θ -type carbides, the volume fraction of the θ -type carbides is 70% or more of all the carbides, and the average grain size is 45 to 80nm.

5. The 1200MPa battery pack steel for the new energy automobile according to claim 1, wherein the tensile strength of the battery pack steel is more than 1200MPa, the yield strength is 900-1050MPa, the elongation is more than or equal to 12%, the hole expansion rate is more than or equal to 55%, the isothermal 5min yield failure is less than 200MPa at 400 ℃, and the isothermal 5min yield failure is less than 500MPa at 800 ℃.

6. A method for preparing the 1200MPa battery pack steel for the new energy automobile according to any one of claims 1-5, comprising smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing, galvanization and finishing; the method is characterized in that:

And (3) hot rolling:

the heating temperature is 1260-1310 ℃, and the isothermal temperature is 120-160 min; the initial rolling temperature is 1080-1130 ℃, the final rolling temperature is 900-950 ℃, then ultra-fast cooling and laminar cooling are carried out, the ultra-fast cooling speed is 5-8 ℃/s, the laminar cooling speed is 2-3 ℃/s, and the coiling temperature is 450-490 ℃;

Acid washing:

The pickling speed is 90-110 m/min;

Cold rolling:

the cold rolling reduction rate is 40% -58%;

Continuous annealing galvanization:

① The isothermal temperature is 920-950 ℃ and the isothermal time is 10-40 s,

② The slow cooling temperature is 720-780 ℃, the slow cooling speed is controlled to be 1-3 ℃/s,

③ Rapidly cooling to below 500 ℃ at a cooling rate of more than 45 ℃/s,

④ Directly quenching in water at 80-95 ℃,

⑤ Reheating: heating to 450-470 ℃ at a heating rate of 10-20 ℃/s, and keeping the isothermal time at 10-18 s,

⑥ Alloying hot galvanizing: the steel strip is galvanized by a zinc pot and enters an alloying furnace for isothermal treatment, the alloying temperature is 480-490 ℃, and the isothermal treatment lasts for 20-30 s.

7. The method for preparing the 1200MPa battery pack steel for the new energy automobile, which is disclosed in claim 6, is characterized in that: in the continuous casting process, the tundish temperature is 1530-1560 ℃, and the casting blank drawing speed is 0.6-0.8 m/min.