Steel for battery pack shell of new energy automobile and preparation method of steel

Abstract

The invention relates to the technical field of advanced materials for new energy automobiles, in particular to steel for a battery pack shell of a new energy automobile and a preparation method thereof.The steel comprises, by mass, 0.26%-0.29% of C, 0.45%-0.65% of Si, 2.2%-2.4% of Mn, 0.65%-0.85% of Cr, 0.02%-0.03% of Nb, 0.02%-0.03% of Ti, 0.0025%-0.0050% of B, smaller than or equal to 0.005% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.0035% of N, smaller than or equal to 0.003% of O and the balance Fe and inevitable impurities; wherein Nb + Ti is not less than 0.045% and not more than 0.055%, and Mn / Cr is not less than 2.6% and not more than 3.5%. The yield strength of the steel is 950-1150 MPa, the tensile strength is larger than or equal to 1300 MPa, and the percentage elongation after fracture is larger than or equal to 12.5%.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of advanced materials for new energy vehicles, in particular to a steel for a battery pack shell of a new energy vehicle and a preparation method thereof. BACKGROUND

[0002] The battery pack, as the core power source of a new energy vehicle, is closely related to the three key directions of safety, lightweight, and reliability, which are highly concerned in the field of new energy vehicle research. The battery pack shell, mainly including a lower shell, an upper cover plate, a frame, a cross beam, and a longitudinal beam, is a load-bearing and protective structure of the battery system. It not only affects the safety and energy consumption of the vehicle, but also bears the sealing, protection, and support of the battery module. The shell material not only needs to maintain high strength and stability under long-term service conditions, but also needs to provide effective protection in extreme heat events. It is a key to passive safety design of new energy vehicles. Ultra-high strength steel material, with its non-combustibility, high melting point, and the ability to work with thermal insulation materials, has become one of the key materials that achieve the best balance between safety, cost, and processability. Therefore, in recent years, the steel for battery pack shell has become the focus of research and development of steel enterprises.

[0003] Under actual use conditions, the steel for battery pack shell usually faces three types of high-temperature scenarios, and its heat resistance directly determines the safety level of the vehicle. The first type is the normal working scenario, where the temperature in the shell cavity reaches 50-90℃ during battery charging and discharging. The steel needs to maintain strength stability to avoid creep deformation caused by long-term heat vibration, and to ensure sealing and assembly precision. The second type is the vehicle fire scenario, where the ambient temperature is 300-500℃ within the 0-5min escape golden period after the fire. The shell is affected by high-temperature conduction, and the material needs to maintain heat resistance stability to prevent structural softening and collapse, and to avoid secondary disasters. The third type is the thermal runaway extreme scenario, where the battery cell failure can spray high-temperature flames and gas flow above 600℃. The shell needs to maintain structural integrity within 5-10min to prevent rupture and leakage, and to provide time for escape and emergency disposal. Based on the safety design requirements of the above multiple scenarios, the steel for battery pack shell needs to have full-level strength attenuation resistance, i.e. maintaining high yield strength and thermal stability in the temperature range from room temperature to 800℃, so as to ensure structural safety under multiple working conditions and meet the safety requirements of normal working, fire, and thermal runaway high-temperature scenarios.

[0004] The publication CN117187682A discloses a 1200MPa battery pack steel for new energy vehicles and a preparation method thereof, the main chemical components of which are (in percentage by weight): 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%, and the balance of Fe and inevitable impurities; the preparation process includes smelting, continuous casting, hot rolling, pickling, cold rolling, continuous annealing and galvanizing, and the performance of the steel is: tensile strength of 1200MPa or more, yield strength of 900-1050MPa, elongation of ≥12%, and hole expansion rate of ≥55%. However, the scheme still has the following deficiencies: the high Al content is easy to cause the formation of Al2O3 inclusions in the casting blank, resulting in a decrease in toughness; the upper limit of the P element content is 0.015%, and the P element is a harmful element in steel, which is easy to segregate at the grain boundary, causing material embrittlement; the high-temperature strength stability is insufficient, and the yield strength attenuation at 400°C and 800°C is still large, which is difficult to meet the safety requirements of fire and thermal runaway scenarios.

[0005] Therefore, there is an urgent need for a new battery pack shell steel that balances high strength, good formability, and high-temperature yield strength stability to improve the intrinsic safety of new energy vehicles under extreme thermal working conditions. SUMMARY

[0006] In view of the technical problem of insufficient high-temperature strength stability of the battery pack shell steel in the prior art, the present application provides a battery pack shell steel for new energy vehicles and a preparation method thereof. By optimizing the alloy component ratio and process parameter collaborative design, the yield strength attenuation rate of the steel at different temperature intervals is effectively reduced, and the overall safety and thermal stability of the material are improved. The microstructure of the steel plate of the present application is composed of >85% tempered martensite and (9-14)% grain boundary recrystallized ferrite, and the grain size of the grain boundary recrystallized ferrite is 0.5-2.0μm. This microstructure structure provides high strength while having excellent ductility, achieving a balance between high strength and high toughness. And through the hierarchical strength performance evaluation results of the battery system under the three major high-temperature scenarios that the battery pack shell steel may face, such as normal working, vehicle fire, and thermal runaway (extreme), the safety and reliability of the battery pack shell are greatly improved.

[0007] The technical scheme of the present application is as follows: In a first aspect, the present application provides a kind of battery package shell steel of new energy vehicle, the component of the steel is as follows according to mass percentage: C 0.26%-0.29%, Si 0.45%-0.65%, Mn 2.2%-2.4%, Cr 0.65%-0.85%, Nb0.02%-0.03%, Ti 0.02%-0.03%, B 0.0025-0.0050%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.003%, the balance is Fe and inevitable impurities; Wherein, satisfy 0.045%≤Nb+Ti≤0.055%, and 2.6≤Mn / Cr≤3.5.

[0008] The steel component design reason of the present application is: C element is the strengthening element in high-strength steel, and the C content in the present application is controlled to be 0.26%-0.29%, which provides the basis for the formation of martensite, ensures the high strength of the steel plate, and prevents the decrease of weldability and cold brittle risk caused by too high C; Si is a typical solid solution strengthening element, and the Si content in the present application is controlled to be 0.45%-0.65%, which inhibits cementite precipitation and stabilizes the tempered martensite structure; Mn is a conventional strengthening and hardenability element, and the Mn content in the present application is controlled to be 2.2%-2.4%, which significantly improves the hardenability in cooperation with C, promotes the formation of tempered martensite structure, and reduces the ferrite content during hot rolling and annealing to ensure the stability of strength. Excessive Mn will lead to excessive cold rolling deformation resistance and segregation, and the present application controls the Mn / Cr ratio in the range of 2.6~3.5 to achieve a comprehensive balance of strength, plasticity and hot workability; Cr element has solid solution strengthening and oxidation resistance, and the Cr content in the present application is controlled to be 0.65%-0.85%, which improves the hardenability in cooperation with Mn, and significantly improves the oxidation stability of the steel plate in high temperature environment; Cr can also improve the edge crack sensitivity and enhance the surface quality during hot rolling. Excessive Cr content will lead to temper brittleness, and the present application limits the Mn / Cr ratio to effectively prevent this problem; Nb and Ti refine the grains by forming carbonitride (NbC, TiN, Ti(C,N)), and inhibit the growth of austenite grains. The Nb content is controlled to be 0.02%~0.03%, the Ti content is controlled to be 0.02%~0.03%, and the total of the two is 0.045%~0.055%. This ratio ensures that the amount of precipitated phase is moderate, which can stabilize the austenite grain boundary without forming coarse inclusions; B is an important element to improve the hardenability of steel, the content of B in the application is controlled in 0.0025%~0.0050%, B is segregated at the austenite grain boundary, inhibits the formation of grain boundary ferrite, promotes the continuous nucleation of martensite, improves the uniformity of the structure, and further improves the high temperature strength retention rate of the steel; P and S are harmful elements, which are easy to form brittle compounds or sulfide inclusions at the grain boundary, weaken the toughness and ductility of the steel plate. The application strictly controls P≤0.005%, S≤0.003%, effectively inhibits the grain boundary embrittlement and hot brittle crack.

[0009] The application realizes the comprehensive performance of high strength, high toughness and low attenuation through the synergistic optimization design of C, Mn, Cr, Mo, Nb, Ti and B, especially through the double constraint balance of Nb+Ti and Mn / Cr.

[0010] Further, the microstructure of the steel includes, in volume percentage: tempered martensite>85%, grain boundary recrystallized ferrite 9%~14%. The martensite provides high strength for the matrix, the tempering treatment eliminates internal stress and avoids brittleness, and the grain boundary recrystallized ferrite is distributed in the martensite grain boundary, which improves the toughness of the material and relieves stress concentration.

[0011] Further, the grain boundary recrystallized ferrite grain size is 0.5~2.0μm.

[0012] Further, the steel satisfies at least one of the following performances: yield strength is 950~1150MPa, tensile strength≥1300MPa, yield strength ratio is 0.75~0.80, and elongation after fracture≥12.5%.

[0013] Further, the steel satisfies at least one of the following performances: 100℃ isothermal for 10min, yield strength attenuation rate<4%; 400℃ isothermal for 10min, 10%<yield strength attenuation rate<20%; 800℃ isothermal for 10min, 30%<yield strength attenuation rate<45%.

[0014] In the second aspect, the application provides a preparation method of a battery pack shell steel for a new energy automobile, which comprises smelting, hot rolling, cover annealing, acid rolling, continuous annealing and galvanizing processes, (1) smelting process: According to the mass percentage of the chemical composition, the slab is obtained by smelting and casting; (2) hot rolling process: S1, furnace loading: the above slab is subjected to hot charging and hot feeding process system, and is sent to the hot rolling process within 4h after the slab production is completed, and is loaded into the heating furnace; S2, heating: the slab heating temperature is set to 1220~1250℃, and the holding time is 2.5~3.5h; S3, rolling: final rolling temperature 900~930℃, the slab is rolled to 2.5~5.0mm; S4, cooling and holding: after rolling, three-stage cooling mode is adopted, the first stage is cooled to 690~730℃ at a cooling rate of >60℃ / s; the second stage is naturally cooled to 660~680℃; the third stage is cooled to 510~535℃ at a cooling rate of 15~20℃ / s, and then coiled, and the hot-rolled steel coil is placed in a holding pit for slow cooling, and the slow cooling time is 72h. The three-stage controlled cooling strategy realizes the controlled transformation of austenite to tempered martensite through step-by-step cooling: the first stage prevents the generation of pearlite, the second stage promotes the precipitation of a small amount of grain boundary ferrite, and the third stage refines the austenite grains and inhibits the transition of bainite. Slow cooling for 72h fully releases the organizational stress, ensuring the stability of the grain and the surface quality.

[0015] (3) Cover annealing process: The above hot-rolled coil is subjected to cover annealing, wherein the temperature of heating 1 is set to 450~470℃; the temperature of heating 2 is set to 670~690℃; the holding section is set to 670~690℃, and the holding time is set to 10~12h; (4) Pickling and cold rolling process: The above hot-rolled steel coil is pickled to remove the oxides on the surface of the strip, and then the strip is cold rolled with a reduction of 50~65%; (5) Continuous annealing and galvanizing process: The annealing temperature is set to 850~870℃, and the strip is rapidly cooled to Ms~Mf temperature 280~310℃ at a cooling rate of ≥50℃ / s for 30~60s; then the strip is rapidly heated to 460~470℃, and the zinc pot temperature is 460~470℃; after coming out of the zinc pot, the galvanized steel strip is cooled to 210~230℃ and is finished. Rapid cooling ensures sufficient martensite nucleation and prevents excessive ferrite precipitation; the reheating stage stabilizes the carbide distribution and promotes the formation of tempered martensite; the finishing micro-deformation can release residual stress and improve the surface flatness and the adhesion of the coating.

[0016] Further, the first stage adopts ultrafast cooling water cooling mode, the second stage adopts air cooling mode, and the third stage adopts laminar flow mode.

[0017] Further, the reflectivity of the surface of the strip after pickling is ≥70%.

[0018] Further, the strip is cold rolled to a thickness of 1.0~2.0mm.

[0019] Further, the elongation of the finishing is 0.7~0.9%.

[0020] The beneficial effects of the present application are: (1) The present application obtains a new energy automobile battery pack shell steel through alloy and process design, the microstructure is composed of >85% tempered martensite and (9%-14%) grain boundary recrystallized ferrite, and the grain size of the grain boundary recrystallized ferrite is 0.5-2.0 mu m. The yield strength is 950-1150 MPa, the tensile strength is greater than or equal to 1300 MPa, the yield strength ratio is 0.75-0.80, the elongation after fracture is greater than or equal to 12.5%, the high strength and good formability are considered, and the strict requirements of the new energy automobile battery pack shell steel are met.

[0021] (2) The present application realizes the thermal stabilization of high-temperature structure by controlling the alloy ratio and multi-stage cooling-tempering system. The layered strength performance evaluation results are excellent under three high-temperature scenarios that the battery system may face in normal work, automobile fire and thermal runaway (extreme), which greatly improves the safety of the battery pack shell, wherein the yield strength attenuation rate is less than 4% at 100 DEG C isothermal for 10 min, the material still maintains high bearing capacity, effectively inhibits the creep deformation under long-term heating and vibration, and ensures the sealing performance and dimensional stability of the shell; the yield strength attenuation rate is 10%-20% at 400 DEG C isothermal for 10 min, which can maintain the structural strength in the early stage of fire to prevent the shell from softening and collapsing rapidly; the yield strength attenuation rate is 30%-45% at 800 DEG C isothermal for 10 min, which can maintain the basic structure of the shell and avoid the spread of thermal runaway caused by shell rupture during the combustion process. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description, and obviously, other drawings can be obtained by those skilled in the art without creative labor on the premise of these drawings.

[0023] Figure 1 It is the microstructure morphology diagram of the new energy automobile battery pack shell steel prepared in embodiment 1-I. DETAILED DESCRIPTION

[0024] In order to make the person skilled in the art better understand the technical solutions in the present application, the following will combine the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the protection scope of the present application.

[0025] The chemical composition of the billet used in embodiments 1-8 of the present application is: C 0.26%-0.29%, Si 0.45%-0.65%, Mn 2.2%-2.4%, Cr 0.65%-0.85%, Nb 0.02%-0.03%, Ti 0.02%-0.03%, B 0.0025-0.0050%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.003%, and 0.045%≤Nb+Ti≤0.055%, 2.6≤Mn / Cr≤3.5, the balance being Fe and unavoidable impurities.

[0026] The chemical composition of the billet in specific embodiments 1-8 is shown in Table 1.

[0027] Table 1 Chemical composition and mass percentage of billet in embodiments 1-8

[0028] The steel for a battery pack shell of a new energy vehicle in embodiments 1-8 is prepared according to the following specific method.

[0029] (1) Smelting process: smelt according to the mass percentage of the chemical composition, and cast to obtain a slab; (2) Hot rolling process: S1, furnace charging: the above slab is subjected to a hot charging and hot feeding process system, and is charged into a heating furnace within 4h after the slab production is completed; S2, heating: the slab heating temperature is set to 1220-1250℃, and the holding time is 2.5-3.5h; S3, rolling: the final rolling temperature is 900-930℃, and the slab is rolled to 2.5-5.0mm; S4, cooling and holding: after rolling, a three-stage cooling method is adopted, the first stage adopts an ultra-fast cooling water cooling mode, and is cooled to 690-730℃ at a cooling rate of >60℃ / s; the second stage adopts an air cooling mode, and is naturally cooled to 660-680℃; the third stage adopts a laminar flow mode, and is cooled to 510-535℃ at a cooling rate of 15-20℃ / s. Then, coiling is performed, and the hot-rolled steel coil is placed in a holding pit for slow cooling, and the slow cooling time is 72h.

[0030] (3) Cover annealing process: the above hot-rolled coil is subjected to cover annealing. The temperature of heating 1 section is set to 450-470℃; the temperature of heating 2 section is set to 670-690℃; the holding section is set to 670-690℃, and the holding time is set to 10-12h.

[0031] (4) Pickling process: the hot-rolled steel coil is pickled to remove the oxides on the surface of the strip, and then the strip is cold-rolled to 1.0-2.0 mm with a reduction of 50-65%, and the surface reflectivity of the strip after pickling is ensured to be ≥70%.

[0032] (5) Continuous annealing and galvanizing process: the annealing temperature is set to 850-870℃, and the cooling rate is ≥50℃ / s to rapidly cool to (Ms-Mf) temperature 280-310℃, and the isothermal time is 30-60s; then the temperature is rapidly raised to 465±5℃ by induction heating, and the zinc pot temperature is 465±5℃; after coming out of the zinc pot, the galvanized steel strip is cooled to 210-230℃ by moving the air box; the skin passing elongation is set to 0.7-0.9%.

[0033] The specific process parameters in Examples 1-8 are shown in Tables 2 and 3.

[0034] Table 2 Specific process parameters of the hot-rolling process and the batch annealing process of Examples 1-8

[0035] Table 3 Specific process parameters of the pickling process, continuous annealing and galvanizing process of Examples 1-8

[0036] The sample preparation and mechanical property testing method at room temperature is in accordance with the national standard GB / T 228.1-2021 "Metallic Materials Tensile Testing Part 1: Room Temperature Test Method", and the mechanical property testing method at high temperature is in accordance with the national standard GB / T 4338-2006 "Metallic Materials High Temperature Tensile Testing Method".

[0037] The mechanical properties and microstructure detection results of Examples 1-8 are shown in Table 4.

[0038] Table 4 Mechanical properties and microstructure detection results of Examples 1-8

[0039] As can be seen from Table 4, the battery pack shell steel prepared in Examples 1-8 has a yield strength of 950-1150MPa, a tensile strength of ≥1300MPa, a yield strength ratio of 0.75-0.80, an elongation after fracture of ≥12.5%, a yield strength attenuation rate of <4% at 100℃ for 10min, a yield strength attenuation rate of 10%< yield strength attenuation rate <20% at 400℃ for 10min, and a yield strength attenuation rate of 30%< yield strength attenuation rate <45% at 800℃ for 10min.

[0040] Figure 1The microstructure morphology analysis result of the new energy automobile battery pack shell steel prepared in example 1-I shows that the new energy automobile battery pack shell steel obtained by adopting the technical scheme is composed of more than 85% tempered martensite and (9-14)% grain boundary recrystallized ferrite, and the grain size of the grain boundary recrystallized ferrite is 0.5-2.0 μm.

[0041] Although the present application has been described in detail by preferred embodiments, the present application is not limited to this. Any modification or replacement of the embodiments of the present application made by those skilled in the art without departing from the spirit and essence of the present application shall be within the scope of the present application. Any modification or replacement made by those skilled in the art within the technical range disclosed by the present application shall be within the protection scope of the present application.

Claims

1. A steel for a battery pack case of a new energy vehicle, characterized by, The steel has the following components in percentage by mass: C 0.26%-0.29%, Si 0.45%-0.65%, Mn 2.2%-2.4%, Cr 0.65%-0.85%, Nb 0.02%-0.03%, Ti 0.02%-0.03%, B 0.0025-0.0050%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.003%, and the balance of Fe and inevitable impurities. Wherein, 0.045%≤Nb+Ti≤0.055%, and 2.6≤Mn / Cr≤3.

5.

2. The steel for a battery pack case of a new energy vehicle according to claim 1, characterized by, The microstructure of the steel includes, in percentage by volume: tempered martensite >85%, grain boundary recrystallized ferrite 9%-14%.

3. The steel for a battery pack case of a new energy vehicle according to claim 2, characterized by, The grain boundary recrystallized ferrite has a grain size of 0.5-2.0 μm.

4. A steel for a battery pack case of a new energy vehicle according to any one of claims 1 to 3, characterized in that, The steel satisfies at least one of the following properties: yield strength 950-1150 MPa, tensile strength ≥1300 MPa, yield strength ratio 0.75-0.80, and elongation after fracture ≥12.5%.

5. The steel for a battery pack case of a new energy vehicle according to claim 1, characterized by, The steel satisfies at least one of the following properties: 100℃ isothermal for 10 min, yield strength attenuation rate <4%; 400℃ isothermal for 10 min, 10%< yield strength attenuation rate <20%; 800℃ isothermal for 10 min, 30%< yield strength attenuation rate <45%.

6. A method of producing a steel for a battery pack case of a new energy vehicle according to any one of claims 1 to 5, characterized by, The process includes smelting, hot rolling, batch annealing, pickling, continuous annealing and galvanizing processes, (1) Smelting process: The steel is smelted according to the chemical components in percentage by mass, and a slab is obtained by casting; (2) Hot rolling process: S1, furnace charging: the slab is subjected to hot charging and hot feeding process system, and is charged into a heating furnace within 4h after the completion of slab production; S2, heating: the slab heating temperature is set to 1220-1250℃, and the holding time is 2.5-3.5h; S3, rolling: the final rolling temperature is 900-930℃, and the slab is rolled to 2.5-5.0mm; S4, cooling and holding: after rolling, a three-stage cooling method is adopted, the first stage is cooled to 690-730℃ at a cooling rate of >60℃ / s; the second stage is naturally cooled to 660-680℃; the third stage is cooled to 510-535℃ at a cooling rate of 15-20℃ / s, and then coiled, and the hot-rolled steel coil is placed in a holding pit for slow cooling, and the slow cooling time is 72h; (3) Batch annealing process: The hot-rolled coil is subjected to batch annealing, wherein the heating 1 stage temperature is set to 450-470℃; the heating 2 stage temperature is set to 670-690℃; the holding stage is set to 670-690℃, and the holding time is set to 10-12h; (4) Pickling process: The hot-rolled steel coil is pickled to remove the oxides on the surface of the strip, and then the strip is cold-rolled with a reduction of 50-65%; (5) Continuous annealing and galvanizing process: The annealing temperature is set to 850-870℃, the cooling rate is ≥50℃ / s, the temperature is rapidly cooled to Ms~Mf temperature 280-310℃, the isothermal time is 30-60s; then the strip steel is rapidly heated to 460-470℃, and the entering zinc pot temperature is 460-470℃; after exiting the zinc pot, the galvanized steel strip is cooled to 210-230℃ and finished.

7. The method of claim 6, wherein the steel for a battery pack case of a new energy vehicle is prepared by the steps of: preparing a steel plate; and performing a heat treatment on the steel plate. The three-stage cooling mode is as follows: The first stage adopts ultrafast cooling water cooling mode, the second stage adopts air cooling mode, and the third stage adopts laminar flow mode.

8. The method of claim 6, wherein the steel for a battery pack case of a new energy vehicle is prepared by the steps of: preparing a steel plate; and performing a heat treatment on the steel plate. The reflectivity of the strip steel surface after pickling is ≥70%.

9. The method of claim 6, wherein the steel for a battery pack case of a new energy vehicle is prepared by the steps of: preparing a steel plate; and performing a heat treatment on the steel plate. The strip steel is cold rolled to a thickness of 1.0-2.0mm.

10. The method of claim 6, wherein the steel for a battery pack case of a new energy vehicle is prepared by the steps of: preparing a steel plate; and performing a heat treatment on the steel plate. The finishing elongation is 0.7-0.9%.

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