800MPa grade thin gauge hot-rolled steel for automobile body and preparation method thereof

Abstract

The present invention belongs to the field of steel materials technology, specifically an 800 MPa-grade thin-gauge hot-rolled steel for automobile bodies and a method for preparing the same. The chemical composition and weight percentage of the steel are as follows: C 0.05-0.09%, Si 0.15-0.25%, Mn 1.40-1.60%, Al 0.015-0.060%, Nb 0.02-0.03%, V 0.03-0.04%, Ti 0.10-0.11%, Cr 0.20-0.30%, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.0050%, O ≤ 0.0030%, with the balance being Fe and unavoidable inclusions. The hot-rolled steel for automobile bodies of the present invention not only has high strength, but also maintains a thin-gauge profile and exhibits certain wear resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel materials, and relates to an automobile compartment plate, which is a hot-rolled high-strength steel with a tensile strength of 800 MPa, and is a 2.0-5.0 mm thin commercial vehicle compartment plate used for transporting sand and stones. Background Art

[0002] Automobile body panels are the primary contact components supporting cargo, gravel, livestock, and other objects. They directly impact the vehicle's service life and driving safety. The manufacturing (stamping) requirements and service conditions of automobile body panels dictate that the steel grade must possess certain strength and toughness, good cold bending properties, excellent wear resistance, and possess good shape and high dimensional accuracy.

[0003] With the introduction of the concepts of "carbon peak and carbon neutrality" in recent years, the construction of a resource-saving and environmentally friendly society has received widespread attention. Among them, the automobile industry has a huge impact. my country has put forward higher requirements for reducing automobile weight, reducing fuel consumption, improving the strength of automobile structural parts and safety performance. Automobile steel has gradually developed towards lightweight and high-strength. At present, 700MPa-grade hot-rolled automobile high-strength steel generally adopts Nb and Ti micro-alloying component system, combined with the corresponding TMCP controlled rolling and controlled cooling technology, to develop and produce high-strength automobile body steel with highly matched strength and toughness and good welding performance. It can effectively reduce the weight of the vehicle and achieve lightweighting of the vehicle. For every 0.1mm thinning of the automobile steel plate thickness, the body weight is reduced by about 10-12%, so the demand for thin-gauge (2.0-5.0mm) high-strength automobile body steel is the greatest.

[0004] However, the plate shape or flatness of thin-gauge high-strength steel has always troubled manufacturers. At the same time, the strengthening effects of Nb and Ti have a certain limited effect on the strength above 800MPa, and the cost of Nb is relatively high. Therefore, there is an urgent need to develop an 800MPa-grade hot-rolled high-strength steel with an optimal plate shape and suitable for production. Summary of the Invention

[0005] In order to solve the problem of thin-gauge high-strength steel plate shape, the present invention provides an 800MPa-grade thin-gauge hot-rolled automobile body steel, which adapts to the new development direction of the low-carbon, energy-saving and environmentally friendly transportation industry, realizes vehicle lightweighting, energy conservation and emission reduction, reduces energy consumption, improves safety, and takes into account cost and production difficulty at the same time, and has high practicality.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0007] The present invention provides an 800MPa-grade thin-gauge hot-rolled steel for automobile bodies. The chemical composition of the steel and the weight percentage thereof are as follows: C 0.05-0.09%, Si 0.15-0.25%, Mn 1.40-1.60%, Al 0.015-0.060%, Nb 0.02-0.03%, V 0.03-0.04%, Ti 0.10-0.11%, Cr 0.20-0.30%, P≤0.015%, S≤0.003%, N≤0.0050%, O≤0.0030%, and the balance is Fe and unavoidable inclusions.

[0008] In the above technical solution, further, the chemical composition of the steel and its weight percentage are: C 0.06-0.09%, Si 0.15-0.25%, Mn 1.40-1.60%, Al 0.020-0.050%, Nb 0.02-0.03%, V 0.03-0.04%, Ti 0.10-0.11%, Cr 0.20-0.30%, P≤0.015%, S≤0.003%, N 0.0015-0.0045%, O 0.0010-0.0020%, and the balance is Fe and unavoidable inclusions.

[0009] In the above technical solution, further, the specification of the steel is 2.0mm to 5.0mm.

[0010] In the above technical solution, further, the method includes steelmaking, hot rolling, annealing, and leveling; the steelmaking includes raw material pretreatment process, converter smelting process, refining process and continuous casting process; the hot rolling process includes heating furnace heating process, rough rolling process, finishing rolling process, cooling coiling and storage; the continuous casting process performs protective casting throughout; in the heating furnace heating process, the temperature of the first heating section is 1280-1300°C, the temperature of the second heating section is 1330-1350°C, and the temperature of the soaking section is 1220~1280°C; in the finishing rolling process, the frame roll shifting coefficient F5 is 0.2, F6 is 0.15, the rolls are shifted in advance, and the final rolling temperature is 900~950°C; the cooling adopts intermittent cooling method.

[0011] In the above technical solution, further, the annealing adopts hood annealing, and the annealing temperature is 500-600°C.

[0012] In the above technical solution, further, the flat rolling force is 4000-5000KN.

[0013] In the above technical solution, further, in the heating process of the heating furnace, the total time in the furnace is ≥180 minutes.

[0014] In the above technical solution, further, the coiling temperature is 600-640°C.

[0015] In the above technical solution, further, in the finishing rolling process, the F1 inlet temperature is ≤1080°C, the F1 reduction ratio is ≥35%, and the F2 reduction ratio is ≥25%.

[0016] In the above technical solution, further, in the refining process, calcium silicon wire calcium treatment is adopted, and 400-500 meters of CaSi wire is fed to fully spheroidize the inclusions.

[0017] The main elements have the following functions:

[0018] C: 0.05-0.09wt%, the carbon content in the steel increases, the yield point and tensile strength increase, but the plasticity and impact resistance decrease. Too high carbon content will affect the cold forming performance and welding performance, so low carbon design is adopted.

[0019] Si: 0.15-0.25wt%. Si has a strong affinity with O and is a strong deoxidizing element. It exists in the steel in the form of solid solution. Si can improve the strength, fatigue limit, corrosion resistance and wear resistance of steel. However, if the Si content is too high, oxides are easily generated during hot rolling, which reduces the surface quality of the steel.

[0020] Mn: 1.40-1.60 wt%. Exists in solid solution in steel, it is a solid-solution strengthening element that increases the strength of ferrite. Mn in low-carbon steel significantly improves strength. However, Mn and S easily form MnS plastic inclusions, which elongate along the rolling direction during hot rolling, deteriorating the steel's formability. Increasing Mn content also causes segregation defects, so it should not be too high.

[0021] Nb: 0.02-0.03wt%, which has extremely strong binding force with carbon, nitrogen and oxygen, and forms corresponding extremely stable compounds with them, thereby refining the grains and further increasing the strength of the steel without significantly reducing the toughness.

[0022] V: 0.03-0.04%. It has a strong binding force with carbon and nitrogen, precipitating in ferrite at the austenite grain boundaries. During rolling, it prevents austenite recrystallization and inhibits grain growth, thereby refining the ferrite grains and improving strength and toughness. It is well known that V (C, N) precipitates in small amounts in austenite, primarily in ferrite. Compared with Nb and Ti, it has a stronger precipitation strengthening effect. Increasing the N content and applying a certain amount of deformation can promote the precipitation of some V (C, N) in austenite, resulting in grain refinement and improved toughness. The remaining V element precipitates in ferrite, producing a precipitation strengthening effect.

[0023] Ti: 0.10-0.11wt%, Ti has the effects of grain refinement and precipitation strengthening. At high temperatures, it can dissolve into austenite and retard the (γ→α) phase transformation. The TiN and TiC precipitated in the steel can prevent grain growth in the austenite and hinder the recrystallization of the deformed austenite, thereby refining the grains. At the same time, the precipitated "effective Ti" TiC has a strong strengthening effect. my country has abundant Ti content and low price, but when the content exceeds 0.10%, the strengthening effect is not obvious.

[0024] Cr: 0.20-0.30wt%, added as an alloying element to prevent the formation of Fe3C. As a medium-strength carbide-forming element, chromium can significantly improve strength, hardness and wear resistance.

[0025] N: not more than 0.0050wt%. Too high N content will affect the formation of effective Ti and affect the strength.

[0026] O: not more than 0.0030wt%, low O control, reducing inclusions, reducing defects, and improving toughness and plasticity.

[0027] P: not more than 0.015wt%. Generally speaking, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates welding performance, reduces plasticity, and worsens cold bending performance.

[0028] S: not more than 0.003 wt %. As a harmful element, it makes the steel hot brittle, reduces the ductility and toughness of the steel, easily generates cracks during the rolling process, and is also detrimental to the welding performance.

[0029] Compared with the prior art, the present invention has the following beneficial effects: the hot-rolled steel for automobile bodies innovatively adds the element V, which eliminates the need for rolling in the non-recrystallization zone during the hot finishing rolling stage. This allows for appropriately increased temperatures and reduced mill loads when rolling ultra-high-strength steel. The addition of the element Cr also improves wear resistance for automobile bodies, ensuring a thin-gauge plate shape (flatness) while maintaining high strength while also exhibiting a certain degree of wear resistance. This makes the steel particularly suitable for manufacturing commercial vehicle body panels, extending service life and addressing vehicle weight reduction, environmental protection, and energy conservation issues. This steel adapts to the new development direction of the transportation industry, which is characterized by low carbon, energy-saving, and environmentally friendly characteristics, achieving lightweight vehicles and improving safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Metallographic structure diagram of Example 3;

[0031] Figure 2 Electron microscope image of inclusions in Example 3;

[0032] Figure 3 Example 3 flatness diagram. DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0034] Example

[0035] The method for preparing 800 MPa grade thin gauge hot-rolled automobile body steel of the present invention comprises the following steps:

[0036] 1. Steelmaking

[0037] 1. Raw Material Pretreatment: Desulfurize with a composite desulfurizer containing lime powder and magnesium powder in a ratio of 2.5:1-3.5:1. The lime powder is sprayed at a rate of 20-70 kg / min and the magnesium powder at a rate of 8-14 kg / min. After slag removal, refined scrap steel is added, with the scrap steel accounting for 5%-15% of the molten iron. The sulfur content of the refined scrap steel must not exceed 0.050%. After final pretreatment, sulfur content is guaranteed to be ≤0.0030%.

[0038] 2. Converter smelting process:

[0039] Converter smelting utilizes constant oxygen pressure and variable gun position operation, employing a single-slag smelting and blowing method. The oxygen supply time is 14-18 minutes, ensuring a final carbon content of ≤0.09% and a temperature of 1681-1695°C, all achieved in one go. The ladle is purged with argon before tapping, and slag is blocked in the early stages and later stages to ensure a slag thickness of less than 100mm. The tapping time is guaranteed to be 4-7 minutes, with the taphole shape controlled to prevent loose flow. Deoxidation and alloying begin when 1 / 5 of the steel has been tapped, with ferrochrome added during this process. Alloying is completed by the 4 / 5 mark. Deoxidation is achieved using ferrosilicon and ferroaluminum, with 650kg of ferrosilicon added first, followed by high manganese. The ferroaluminum must be added at least 1 minute after the addition, ensuring that the ladle nitrogen content is ≤25ppm (based on a 170-ton furnace).

[0040] 3. Refining process

[0041] Refining is performed in a single-path LF furnace. Ferrotitanium is added during the refining process, with the mass percentage of Ti in the molten steel at 0.10-0.11%. Activated lime and fluorite are used to create a fluid reducing slag to minimize exposure of the molten steel. Calcium-silicon wire treatment is used, with a 400-500 meter CaSi wire feed to fully spheroidize inclusions and improve product performance. Soft argon purge is performed for ≥10 minutes before the end of the treatment. The temperature of the LF furnace leaving the station for a normal heat is controlled at 1550-1580°C.

[0042] 4. Continuous casting process

[0043] Protected pouring is performed throughout the entire process. Argon is used to purge the tundish before pouring begins. No exposed molten steel is allowed during the pouring process to avoid nitrogen buildup. The shroud is cleaned after each pour and maintained vertically throughout the pouring process. The shroud depth of the tundish is maintained at 200-250mm, and the continuous casting superheat control target is ≤30°C.

[0044] 2. Hot rolling

[0045] 1. Heating process of heating furnace

[0046] The heating furnace has four sections or zones: preheating, primary heating, secondary heating, and soaking. The slab undergoes heating and holding in each section before exiting the furnace for rolling. The primary heating section maintains a temperature of 1280-1300°C, the secondary heating section 1330-1350°C, and the soaking section 1220-1280°C. The high-temperature section is moved forward to bring the secondary heating outlet temperature closer to the target, eliminating temperature differences between the inside and outside of the slab and the watermark, ensuring effective soaking. The total time in the furnace is ≥180 minutes.

[0047] 2. Rough rolling process

[0048] The roughing passes are performed in a 3+3 pattern, with three R1 passes and three R2 passes. This pattern aims to fully refine the austenite and reduce the load on the finishing mill. The R2 finishing temperature is controlled between 1060°C and 1120°C. Water spraying at high temperatures effectively removes surface oxide scale. The intermediate bar thickness is 30-40mm, and the cumulative reduction during the roughing phase is greater than 75%.

[0049] 3. Finishing rolling process

[0050] Finishing rolling requires an F1 inlet temperature of ≤1080°C, an F1 reduction of ≥35%, and an F2 reduction of ≥25%. The roll shifting coefficients for F5 and F6 stands have been optimized, with F5 reduced from 0.35 to 0.2 and F6 from 0.3 to 0.15. Roll shifting is initiated in advance, and the water pressure is maintained at 25-35 MPa. This effectively avoids double-sided waves in the finished product and stabilizes the rolling process. Thin-gauge high-strength steel, due to its greater resistance to deformation, undergoes high-temperature finish rolling at 900-950°C.

[0051] 4. Cooling and coiling

[0052] The cooling mode adopts intermittent cooling, with laminar cooling consisting of four water groups. The first five groups are shut off, and the sixth group is added. Water is then drained alternately every four groups, for a total of two intermittent cooling rows. The coiling temperature is set between 600°C and 640°C, which is conducive to the sufficient precipitation of "effective Ti" and TiC, achieving excellent precipitation strengthening. If the coiling temperature is too low, it is not conducive to plate shape control and the rolling process is difficult to control. If the coiling temperature is too high, the TiC precipitation is insufficient, the cooling rate is insufficient, the precipitation strengthening and grain refinement strengthening effects are not obvious, and the strength is not satisfactory.

[0053] 5. Storage

[0054] The rolled coils are placed in a slow cooling corner for slow cooling, that is, they are placed in a corner away from the warehouse door and surrounded by other high-temperature coils to ensure the plate shape and the release of internal stress.

[0055] 3. Annealing

[0056] Hood annealing is used to fully release internal stress and ensure excellent plate shape. The annealing temperature is 500-600℃.

[0057] 4. Leveling

[0058] The leveling process is carried out on a 2250 leveling mill with a rolling force of 4000-5000KN.

[0059] According to the above preparation method, the ingredients in Table 1 and the processes in Tables 2 and 3 were used to prepare Examples 1 to 6 and Comparative Examples 1 to 2.

[0060] Table 1 Chemical composition / %

[0061]

[0062]

[0063] Table 2 Heating process

[0064] Specifications / mm First stage temperature / ℃ Second adding stage temperature / ℃ Soaking zone temperature / ℃ Time in furnace / min Example 1 2.0*1250 1298 1345 1263 193 Example 2 2.5*1500 1295 1350 1270 182 Example 3 3.0*1500 1290 1338 1245 199 Example 4 3.5*1250 1282 1334 1224 189 Example 5 4.0*1500 1291 1342 1267 191 Example 6 5.0*1500 1283 1336 1225 190

[0065] Table 3 Rolling process / %

[0066] Finishing rolling F1 temperature / ℃ Finish rolling temperature / ℃ Coiling temperature / ℃ Annealing temperature / ℃ Temper rolling force / KN Example 1 1062 932 630 540 4600 Example 2 1070 930 621 545 4500 Example 3 1060 921 604 541 4400 Example 4 1049 925 626 543 4300 Example 5 1062 910 601 560 4250 Example 6 1055 905 614 565 4200

[0067] Performance tests were performed on Examples 1 to 6, and the results are shown in Table 4.

[0068] Table 4 Mechanical properties

[0069]

[0070]

[0071] Example 3 The metallographic structure of 3.0 mm steel plate is shown in Figure 1 , non-metallic inclusions see Figure 2 The metallographic structure is ferrite + pearlite, the grain size is 13.5, and the non-metallic inclusions are D0.5. Figure 3 This is the straightness diagram of Example 3. The performance comparison of Example 3 and Comparative Documents 1 and 2 is shown in Table 5. From the results in Table 5, it can be seen that the steel designed by the present invention not only meets the requirements of high strength and thin specifications, but also meets the requirements of straightness.

[0072] Table 5 Comparative Example Performance

[0073]

Claims

1. An 800MPa grade thin gauge hot-rolled steel for automobile body, characterized in that: The chemical composition of the steel and its weight percentage are: C 0.06-0.09%, Si 0.15-0.25%, Mn 1.40-1.60%, Al 0.020-0.050%, Nb 0.02-0.03%, V 0.03-0.04%, Ti 0.10-0.11%, Cr 0.20-0.30%, P≤0.015%, S≤0.003%, N 0.0015-0.0045%, O 0.0010-0.0020%, and the balance is Fe and unavoidable inclusions; The steel preparation method includes: steelmaking, hot rolling, annealing, and leveling; the steelmaking includes raw material pretreatment, converter smelting, refining, and continuous casting; the hot rolling process includes heating in a heating furnace, rough rolling, finishing rolling, cooling, coiling, and storage; the continuous casting process is performed with protective pouring throughout; in the heating furnace heating process, the temperature of the first heating section is 1280-1300°C, the temperature of the second heating section is 1330-1350°C, and the temperature of the soaking section is 1220-1280°C; in the finishing rolling process, the stand roll shifting coefficient F5 is 0.2, F6 is 0.15, the rolls are shifted in the right direction in advance, and the final rolling temperature is 900-950°C; the cooling method adopts intermittent cooling; The cooling mode adopts intermittent cooling mode. Laminar cooling is carried out in groups of 4. The first 5 groups are closed and the 6th group of water is put into use. Every 4 groups of water are drained alternately, for a total of 2 rows of intermittent cooling mode. The annealing is performed by bell annealing at a temperature of 500-600°C; The coiling temperature is 600-640°C.

2. The 800MPa grade thin gauge hot-rolled automobile body steel according to claim 1, characterized in that: The steel has a specification of 2.0 mm to 5.0 mm.

3. The 800MPa grade thin gauge hot-rolled automobile body steel according to claim 1, characterized in that: The flat rolling force is 4000-5000KN.

4. The 800MPa grade thin gauge hot-rolled automobile body steel according to claim 1, characterized in that: In the heating process of the heating furnace, the total time in the furnace is ≥180 minutes.

5. The 800MPa grade thin gauge hot-rolled automobile body steel according to claim 1, characterized in that: In the finishing rolling process, the F1 inlet temperature is ≤1080°C, the F1 reduction ratio is ≥35%, and the F2 reduction ratio is ≥25%.

6. The 800MPa grade thin gauge hot-rolled automobile body steel according to claim 1, characterized in that: In the refining process, calcium silicon wire is used for calcium treatment, and 400-500 meters of CaSi wire is fed to fully spheroidize the inclusions.

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