Low-cost easily-formed hot-rolled automobile steel and production method thereof

Abstract

The invention relates to low-cost easy-to-form hot-rolled automobile steel and a production method thereof. The steel comprises the following chemical components in percentage by mass: 0.04-0.08% of C, less than or equal to 0.50% of Si, 1.20-1.80% of Mn, less than or equal to 0.030% of Als, 0.040-0.110% of Ti, less than or equal to 0.020% of P, less than or equal to 0.010% of S, less than or equal to 0.0040% of N and the balance of Fe and inevitable inclusions. The metallographic structure of the finished steel plate is ferrite, pearlite and a small amount of bainite. Aiming at high-forming-performance and low-cost production of the 510-700MPa-grade hot-rolled automobile steel, a full-process control technology is adopted, on the premise that a product has good forming performance, the narrow standard requirement is met, and meanwhile the beneficial effects of low cost and the like are achieved; and heating process rhythm control and powerful dephosphorization water spraying are adopted, so that the surface quality and performance of a finished product are ensured, and meanwhile, the production cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of automobile steel production, and particularly relates to a low-cost easy-to-form hot-rolled automobile steel and a production method thereof. BACKGROUND

[0002] With the continuous expansion of the domestic automobile market and the increasing demand of consumers for the safety performance of automobiles, the demand for 510MPa automobile beam steel continues to increase. The wide application of such steel in automobile manufacturing not only improves the safety performance and comfort of automobiles, but also provides the possibility for automobile lightweight design. With the continuous progress of technology, the production technology of 510MPa automobile beam steel products has become relatively stable and mature, and automobile production enterprises have completed the update iteration from 320MPa to 510MPa products or even 700MPa products. However, with the large-scale production of 510MPa automobile beam steel, it is inevitable that there will be large fluctuations in performance and cracking caused by complex forming. Considering that the strength window of 510L steel plate is relatively narrow, the strength of products in the same furnace or same batch can differ by more than 100MPa. In addition, the extreme control of production cost also seriously affects the production efficiency and operating benefits of enterprises, and causes great difficulties for the production and application of downstream automobile production enterprises, such as open warping, bending, roll rebound angle out-of-tolerance, and frequent cracking of products during production.

[0003] 510MPa-700MPa automobile beam steel, as a key material for modern automobile manufacturing, plays an important role in the automobile manufacturing process due to its high strength, good plasticity and toughness, stable coil stability and welding performance, etc.

[0004] A Chinese patent application with the application publication number CN115369307A discloses a "manufacturing method for producing hot-rolled 510L automobile beam steel based on CSP process", which produces hot-rolled 510L automobile beam steel based on CSP process, solving the problem of unstable mechanical properties of 510L strip steel caused by Ti and Nb precipitation strengthening. The specific method is as follows: after pretreatment of molten steel, composite injection desulfurization treatment is carried out; after top and bottom combined blowing smelting in converter, whole-process bottom blowing argon refining treatment after furnace, and LF furnace refining treatment, the chemical composition of the molten steel is controlled; thin slab continuous casting protection casting is carried out, hot charging heating furnace is used, and then high-pressure water descaling is carried out; six-stand continuous rolling is carried out, the out-of-furnace temperature of the slab is controlled by using controlled rolling and controlled cooling process; laminar cooling is carried out, two-end cooling method is adopted to ensure that the strip steel is air-cooled in the 4th group to the 6th group; and underground coiling is carried out. The low-carbon micro-alloy strengthening controlled rolling and controlled cooling process is adopted, trace Ti and Nb are added in the low-carbon steel for strengthening, which can effectively improve the strength and plasticity index of automobile beam steel and realize the stable control of performance. However, the composition thereof adopts Nb and Ti design, which is different from the present application.

[0005] Chinese patent application with publication number CN103451535A discloses "a 510MPa grade automobile beam hot continuous rolling strip steel and its production process", the chemical composition of the steel plate is 0.13-0.18% C, 0.30-0.50% Si, 0.75-1.15% Mn, 0.02-0.06% Alt, P≤0.025%, S≤0.008%, 0.008-0.025% Ti, N≤0.006%, the balance is Fe and inevitable impurities. By fine-tuning C, Si content, reducing Mn content, adding only a small amount of cheap Ti, not adding any other valuable alloy elements, the production cost is significantly reduced, the steel grade can be produced in a reduced amount, energy consumption is saved, cost is reduced, and an automobile beam steel with excellent comprehensive performance is obtained. But its composition uses C content in the peritectic zone design, which is easy to occur liquid level fluctuation, affecting the quality. The present application avoids the peritectic zone design, which is obviously different from the above.

[0006] Chinese patent application with publication number CN112281075A discloses "a method for improving the performance stability of niobium-titanium composite reinforced 700MPa beam steel", the chemical composition of the steel is C: 0.06-0.09%, Mn: 1.50-1.85%, Si: 0.05-0.20%, P: ≤0.015%, S: ≤0.005%, Nb: 0.025-0.060%, Ti: 0.070-0.110%, Mo: ≤0.20%, N: 0.0040-0.0085%, Als: 0.025-0.055%, the balance is Fe and inevitable impurities. Its composition uses a low-silicon design, the focus is not on solving the surface quality problem, but on optimizing and researching stability, which is different from the present application. SUMMARY

[0007] The present application provides a low-cost easy-to-form hot-rolled automobile steel and its production method, aiming at the high forming performance and low-cost production of 510MPa grade hot-rolled automobile steel, using full-process control technology, under the premise of good forming performance of the product, meeting the requirements of a relatively narrow standard, and also having the advantages of low cost, etc.; using heating process rhythm control and strong scale removal water injection to ensure the surface quality and performance of the finished product, while reducing the production cost.

[0008] In order to achieve the above purpose, the present application adopts the following technical scheme: A low-cost hot-rolled automobile steel easy to form, the chemical composition of the steel is as follows: C: 0.04% to 0.08%, Si≤0.50%, Mn: 1.20% to 1.80%, Als≤0.030%, Ti: 0.040% to 0.110%, P≤0.020%, S≤0.010%, N≤0.0040%, and the balance of Fe and inevitable inclusions; the finished steel plate has a microstructure of ferrite + pearlite + a small amount of bainite, wherein the proportion of bainite is ≤5%, the proportion of pearlite is 5% to 15%, and the rest is ferrite.

[0009] The finished products include 700L high-strength steel, 610L high-strength steel and 510L high-strength steel; the mechanical properties of the finished steel plate are as follows: for 700L high-strength steel, yield strength ReH≥610 MPa, tensile strength R m ≥680 MPa; elongation A≥14%; for 610L high-strength steel, yield strength ReH≥500 MPa, tensile strength R m ≥610 MPa; elongation A≥17%; for 510L high-strength steel, yield strength ReH≥355 MPa, tensile strength R m is 510 to 610 MPa; and elongation A≥24%.

[0010] A production method of a low-cost hot-rolled automobile steel easy to form, comprising the following steps: 1) Smelting: deep decarburization and constant oxygen pressure variable lance position operation are adopted in converter smelting, the constant oxygen pressure is 1.3 to 1.4 MPa, the C content at the end of converter is controlled to be 0.03% to 0.04%; single-slag method smelting is adopted for blowing; the oxygen supply time is 14 to 18 min, and the time for tapping is 4 to 7 min; the deoxidization and alloying are started when the amount of molten steel is 1 / 5 of the total amount of molten steel, and all the alloy is added when the amount of molten steel is 4 / 5 of the total amount of molten steel; LF refining is adopted, titanium-iron alloy is added in the LF refining process, the mass content of Ti in the molten steel is 0.040% to 0.110%, and active lime and fluorite are used to form the reducing slag; the soft argon blowing time before the end of LF refining is ≥10 min, and the LF off-site temperature is controlled to be 1540 to 1560℃; 2) Continuous casting: full-process protection pouring is adopted for continuous casting, and weak cooling mode with a specific water consumption of 1.0 to 1.1 L / kg is adopted in the secondary cooling section; light press-down and constant speed mode are adopted for continuous casting, the drawing speed is controlled to be 1.0 to 1.5 m / min, and the continuous casting superheat is ≤25℃; 3) Heating: the cast blank is hot superheated and loaded to ensure that the inlet temperature is ≥500℃; the outlet temperature is ≥1200℃, the heating time is 130 min to 220 min, and the outlet temperature is controlled to be 1200 to 1240℃; the furnace pressure is controlled to be a micro-positive pressure of 10 to 30 Pa, and the air excess coefficient is controlled to be 2.0; 4) Rough rolling: 3+3 rolling mode is adopted, R1 finishing rolling temperature is controlled at 950-1030°C, and R2 finishing rolling temperature is controlled at 980-1070°C; cumulative reduction of rough rolling is >70%; descaling after rough rolling is performed at a temperature of >1180°C and a water pressure of >18 MPa; 5) Finish rolling: hot rolling oil is sprayed on the surfaces of support rollers and work rollers simultaneously; F1 inlet temperature of finish rolling is 950-1030°C, F7 finishing rolling temperature is 850-900°C, finish rolling time is 60-100 s, and descaling after finish rolling is performed at a water pressure of >18 MPa; 6) Cooling and coiling: intermittent cooling mode is adopted, and coiling temperature is set according to product strength grade: for 510L steel, coiling temperature is 580-620°C; for 610L steel and 710L steel, coiling temperature is 400-500°C.

[0011] In the step 1), fine material scrap steel is used for smelting, the molten steel is pretreated first, the mass content of S in the molten steel is ≤0.0030% before tapping, and the slag is cleaned; the ladle is purged with argon before tapping, the slag is blocked in the early stage, and the slag blocking dart is set in the later stage to ensure that the thickness of the ladle slag is <100 mm.

[0012] In the step 2), the tundish is purged with argon before pouring, there is no exposure of the molten steel during pouring, the long nozzle is cleaned after each pouring, and the long nozzle is kept in a vertical state during pouring; 1-5 mm reduction is added at the 5th segment to the 7th segment of the caster.

[0013] In the step 2), according to the measurement standard ±0.3 mm, the arc connection qualified rate of the caster crystallizer and the 0th segment of the caster, the 1st segment of the caster, the 1st segment of the caster and the 2nd segment of the caster, the 7th segment of the caster and the 8th segment of the caster, and each segment is ≥99%, and the arc connection qualified rate of each segment is ≥98%.

[0014] In the step 3), the heating time, heating temperature and furnace atmosphere of each segment in the furnace are controlled as follows: the upper temperature of the preheating segment is 700-1000°C, the lower temperature of the preheating segment is 700-1000°C, and the heating time is 30-40 min; the upper temperature of the first heating segment is 1130-1170°C, the lower temperature of the first heating segment is 1100-1150°C, the heating time is 50-60 min, and a weak oxidation atmosphere with an air-fuel ratio of 1:2.2-1:2.4 is used; the upper temperature of the second heating segment is 1200-1250°C, the lower temperature of the second heating segment is 1180-1230°C, the heating time is 60-70 min, and a weak oxidation atmosphere with an air-fuel ratio of 1:1.9-1:2.1 is used; the upper temperature of the soaking segment is 1180-1230°C, the lower temperature of the soaking segment is 1150-1200°C, the heating time is ≥35 min, and a weak oxidation atmosphere with an air-fuel ratio of 1:2.1-1:2.5 is used.

[0015] The thickness of the intermediate billet after rough rolling is 30-80 mm.

[0016] Compared with the prior art, the present application has the advantages that: For the high forming performance and low cost production of the 510-700MPa grade hot-rolled automobile steel, the full-process control technology is adopted to meet the relatively narrow standard requirement under the premise of good forming performance of the product, and the low cost is also possessed; the heating process rhythm control and strong scale removal water injection are adopted to ensure the surface quality and performance of the product, and the production cost is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a metallographic structure diagram of the low-cost easy-forming hot-rolled automobile steel according to the present application.

[0018] Figure 2 is the iron oxide scale condition of the finished steel plate of Example 3 of the present application.

[0019] Figure 3 is the iron oxide scale condition of the finished steel plate of Comparative Example 3 of the present application. DETAILED DESCRIPTION

[0020] The low-cost easy-forming hot-rolled automobile steel according to the present application has the chemical composition of C: 0.04%-0.08%, Si≤0.50%, Mn: 1.20%-1.80%, Als≤0.030%, Ti: 0.020%-0.110%, P≤0.020%, S≤0.010%, N≤0.0040%, and the balance of Fe and inevitable inclusions, in terms of mass percentage; and the finished steel plate has the metallographic structure of ferrite+pearlite+small amount of bainite, wherein the proportion of bainite is ≤5%, the proportion of pearlite is 5%-15%, and the rest is ferrite (as shown in Figure 1 ).

[0021] The roles of the main chemical elements and the reasons for the content selection in the low-cost easy-forming hot-rolled automobile steel according to the present application are as follows: C: When the mass fraction of carbon in the steel is <0.09%, the forming ability and welding performance of the material are better. When the mass fraction of carbon in the steel is 0.09%-0.16%, the peritectic reaction (L+δ→γ) in the solidification process of the steel liquid is more intense, and the maximum volume shrinkage rate can reach 0.38%, which leads to large linear shrinkage change of the solidification shell. When the linear shrinkage reaches a certain degree, the air gap between the continuous casting billet shell and the mold becomes large, which eventually leads to the occurrence of concave, and the heat flow decreases, and the primary shell becomes thin. Under the action of thermal stress and other stresses, cracks are formed at the concave valley. Therefore, the content of C is controlled to be 0.04%-0.08% in the present application.

[0022] Si: Si and O have strong affinity, belong to strong deoxidizing element, and exist in the form of solid solution in steel; Si can improve the strength, fatigue limit, corrosion resistance and wear resistance of steel, and the effect of adding a certain amount of ferrosilicon for deoxidizing and alloying is better. At the same time, Si is a non-carbide forming element, which can promote the diffusion of carbon elements in ferrite to the inside of austenite during phase transformation, thereby increasing the phase transformation temperature, accelerating the ferrite phase transformation, and being beneficial to increasing the ferrite content in the structure and improving the forming performance of the material. Therefore, the Si content is controlled to be ≤0.50% in the application.

[0023] Mn: exists in the form of solid solution in steel, belongs to solid solution strengthening element, and can improve the strength of ferrite. Mn in low carbon steel has obvious effect on improving strength. However, Mn and S easily form MnS plastic inclusions which are elongated along the rolling direction during hot rolling, thereby deteriorating the forming performance of steel. In addition, with the increase of Mn content, segregation defects will also be caused, therefore, the Mn content is controlled to be 1.20% to 1.80% in the application.

[0024] Als: Al usually precipitates in the form of AlN during high temperature process, and the lower the content of Al and N is, the lower the precipitation temperature is. Since the interface energy at grain boundary is high, it is beneficial to nucleation, therefore, AlN usually precipitates at the grain boundary of austenite, thereby degrading the grain boundary strength; when subjected to tensile stress during the straightening section of continuous casting, it is easy to cause corner crack defects of the casting blank, which is not conducive to the hot passing of the continuous casting blank, therefore, the Als is controlled to be ≤0.030% in the application.

[0025] Ti: Ti is rich in China and has low price. Ti has the effects of fine-grain strengthening and precipitation strengthening, can dissolve into austenite at high temperature to block the (γ→α) phase transformation, and the precipitated TiN and TiC in austenite can prevent grain growth and hinder the recrystallization of deformed austenite, thereby playing the role of refining grains. In terms of hot passing of continuous casting blank, Ti has better effect on the hot plasticity of steel than Nb and V elements, therefore, the Ti content is controlled to be 0.040% to 0.110% in the application.

[0026] N: If the content of N is too high, AlN and sharp corner inclusions TiN will be easily generated by the reaction of N with Al and Ti, thereby affecting the surface quality and performance of the product. The N content is controlled to be ≤0.0040% in the application.

[0027] P: Generally speaking, phosphorus is a harmful element in steel, which can increase the cold brittleness of steel, deteriorate the welding performance, reduce the plasticity, and make the cold bending performance worse. Considering the production cost comprehensively, the P content is controlled to be ≤0.020% in the application.

[0028] S: As a harmful element in steel, S can cause hot brittleness of steel, reduce the ductility and toughness of steel, easily produce cracks during rolling process, and is not conducive to the welding performance. Considering the production cost comprehensively, the S content is controlled to be ≤0.010% in the application.

[0029] The production method of the low-cost easy-to-form hot-rolled automobile steel according to the present application is as follows: I. Steelmaking.

[0030] 1. Pretreatment process; The fine material scrap steel is used as raw material, and after pretreatment, the S mass content in the molten steel in the furnace is ≤0.0030%, and the slag is cleaned.

[0031] 2. Converter process; In the converter smelting, deep decarburization and constant oxygen pressure variable gun position operation are adopted, the constant oxygen pressure is 1.3-1.4 MPa, the C content at the end of the converter is controlled to be 0.03%-0.04%; single slag method is adopted for smelting and blowing, the oxygen supply time is 14-18 min, the ladle is argon purged before tapping, the slag is blocked in the early stage: the slag is blocked by a dart in the late stage of tapping, so as to ensure that the thickness of the ladle slag is less than 100 mm. The tapping time is ensured to be within 4-7 min, the shape of the tapping hole is controlled to avoid scattered flow during tapping. Through the above measures, the oxygen content in the steel can be effectively reduced, thereby indirectly controlling the aluminum content. The deoxidization and alloying are started when the total amount of molten steel is 1 / 5, and the alloying is completed when the total amount of molten steel is 4 / 5.

[0032] 3. Refining process; The LF furnace single-path refining is adopted, titanium iron is added in the LF refining process, and the mass percentage of Ti element in the molten steel is controlled to be 0.040%-0.110%; the LF refining adopts active lime and fluorite to form good fluidity reducing slag, and the molten steel is as much as possible avoided to be exposed; the soft argon blowing time before the end of the LF refining is ≥10 min, and the LF off-site temperature is controlled to be 1540-1560℃.

[0033] 4. Continuous casting process; The arc connection qualified rate of the mold of the casting machine, the zero segment of the casting machine, and the first segment of the casting machine is ≥99%, the arc connection qualified rate between the first segment and the second segment of the casting machine, between the seventh segment and the eighth segment of the casting machine, and between each segment of the casting machine is ≥98% (the measurement standard is ±0.3 mm). The whole continuous casting process is protected pouring. The tundish is purged by argon before pouring, the molten steel is not exposed during pouring, the nitrogen and oxygen increase is avoided, the long nozzle is required to be cleaned after each furnace pouring, and the long nozzle is required to be kept in a vertical state during pouring. The weak cooling mode with a specific water quantity of 1.0-1.1 L / kg and the light press-down mode are adopted in the second cooling segment of the continuous casting, the press-down of 1-5 mm is segmentedly put in the fifth segment to the seventh segment of the casting machine, so as to eliminate the segregation phenomenon; the constant speed is adopted, the drawing speed is controlled to be 1.0-1.5 m / min, and the overheat degree of the continuous casting is controlled to be ≤25℃.

[0034] II. Hot rolling.

[0035] 1. Heating process; For hot superheated billet charging, the billet entering the furnace should have a temperature ≥500℃, and the exit temperature ≥1200℃. The heating time should be greater than 130 min but not exceed 220 min. Strict control should be exercised over the heating time, temperature, and atmosphere in each section of the furnace (as shown in Table 1). The exit temperature should be controlled at 1220±20℃. The furnace pressure should be controlled at a slightly positive pressure of 10–30 Pa, with an excess air coefficient of 2.0. These measures can effectively reduce the FeSi2O4 thickness and gas consumption, reducing gas consumption by more than 1.1 GJ / t of steel.

[0036] Table 1 Heating time, heating temperature and furnace atmosphere for each section of the furnace 2. Rough rolling process; A 3+3 rolling mill pattern is adopted, controlling the final rolling temperature of R1 at 950–1030℃ and R2 at 980–1070℃. The cumulative reduction rate during roughing is greater than 70%, and the intermediate slab thickness is 30–80 mm. The descaling temperature is ≥1180℃; high-temperature water spray descaling better removes iron oxide scale from the steel surface. The water pressure for roughing descaling is above 18 MPa to ensure the surface quality of the slab.

[0037] 3. Finish rolling process; By fully utilizing the effects of hot rolling oil, roll gap cooling, and roll cooling water in improving the surface quality of the work rolls, the conventional method of spraying hot rolling oil only onto the work roll surface has been improved to simultaneously spraying hot rolling oil onto the support rolls and work roll surfaces. This not only allows for the simultaneous use of hot rolling oil and roll gap cooling but also eliminates the need to stop the work roll inlet cooling water. The finishing mill inlet temperature for F1 is 950–1030℃, the finishing mill finishing temperature is 850–900℃, the finishing mill rolling time is 60–100s, and the finishing mill descaling water pressure is above 18MPa.

[0038] 4. Cooling and winding process; Intermittent cooling is employed, with the first seven sets of cooling water using intense cooling (ultra-intensive cooling). The coiling temperature is set according to the product's strength grade: for 510L steel, the coiling temperature is 580–620℃, which facilitates the full precipitation of TiC, resulting in good precipitation strengthening; for 610L and 700L steel, the coiling temperature is 400–500℃. The final microstructure consists of ferrite, pearlite, and a small amount of bainite. The finished products include 700L high-strength steel, 610L high-strength steel, and 610L high-strength steel, all with good formability.

[0039] The mechanical properties of the finished steel plate are as follows: For 700L high-strength steel, the yield strength R eH ≥610MPa, tensile strength R m ≥680MPa; elongation A≥14%; for 610L high-strength steel, yield strength R eH ≥500MPa, tensile strength Rm ≥610MPa; elongation A≥17%; for 510L high-strength steel, yield strength R eH ≥355MPa, tensile strength R m The strength is 510–610 MPa; the elongation A is ≥24%.

[0040] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0041]

Example

[0042] Table 2 Chemical composition of steel in the examples, wt% Table 3 shows the hot rolling process parameters of the steel plate in the examples. Table 4. Test results of mechanical properties of finished steel plates from the examples. Table 5 Chemical composition (wt%) of steels in some examples and comparative examples Table 6 shows the performance comparison results of some examples and comparative examples of finished steel plates. Table 7. Oxide scale inspection results of finished steel plates from Example 3 and Comparative Example 3. Conclusion: The high-strength steel prepared according to the components and the production method can meet the needs of customers for manufacturing complex structural parts. The low-cost (low C+single Ti) design is adopted, and the Si content is increased, so that F+P+small amount of B organization is obtained, and the forming performance (expansion rate) is improved. Meanwhile, multiple steel grades can be produced in the component range, which is beneficial to more flexible production scheduling for the manufacturers, greatly reduces the operation cost of the enterprises, and improves the product competitiveness.

[0043] The above merely describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can make equivalent replacements or changes to the technical solutions and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A low-cost, easily formable hot-rolled automotive steel, characterized in that, The chemical composition of the steel, by mass percentage, is: C: 0.04%–0.08%, Si≤0.50%, Mn: 1.20%–1.80%, Als≤0.030%, Ti: 0.040%–0.110%, P≤0.020%, S≤0.010%, N≤0.0040%, with the balance being Fe and unavoidable inclusions. The microstructure of the finished steel plate consists of ferrite, pearlite, and a small amount of bainite, with bainite accounting for ≤5%, pearlite accounting for 5% to 15%, and the remainder being ferrite.

2. The low-cost, easily formable hot-rolled automotive steel according to claim 1, characterized in that, The finished products include 700L high-strength steel, 610L high-strength steel, and 510L high-strength steel; the mechanical properties of the finished steel plates are as follows: for 700L high-strength steel, the yield strength R... eH ≥610MPa, tensile strength R m ≥680MPa; elongation A≥14%; for 610L high-strength steel, yield strength R eH ≥500MPa, tensile strength R m ≥610MPa; elongation A≥17%; for 510L high-strength steel, yield strength R eH ≥355MPa, tensile strength R m The strength is 510–610 MPa; the elongation A is ≥24%.

3. A method for producing low-cost, easily formable hot-rolled automotive steel as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Smelting: The converter smelting adopts deep decarburization and constant oxygen pressure variable lance position operation. The constant oxygen pressure is 1.3 to 1.4 MPa, and the C content at the end of the converter is controlled at 0.03% to 0.04%. The single slag method is used for smelting and blowing. The oxygen supply time is 14 to 18 minutes, and the tapping time is 4 to 7 minutes. Deoxidation and alloying are started when the steel volume is 1 / 5 and the alloying is completed when the steel volume is 4 / 5. LF refining is adopted, and titanium-iron alloy is added during the LF refining process. The mass content of Ti in the molten steel is 0.040% to 0.110%. Activated lime and fluorite are used to make reducing slag. The soft blowing argon time before the end of LF refining is ≥10 min, and the LF leaving station temperature is controlled at 1540 to 1560℃. 2) Continuous casting: The continuous casting process is protected during pouring. The secondary cooling section adopts a weak cooling mode with a specific water volume of 1.0 to 1.1 L / kg. The continuous casting adopts a light pressure and constant casting speed mode, with the casting speed controlled at 1.0 to 1.5 m / min. The superheat of continuous casting is ≤25℃. 3) Heating: The billet is hot-superheated to ensure that the furnace temperature is ≥500℃ and the furnace temperature is ≥1200℃. The heating time is 130min~220min and the furnace temperature is controlled at 1200~1240℃. The furnace pressure is controlled at a slight positive pressure of 10~30Pa and the excess air coefficient is controlled at 2.

0. 4) Rough rolling: The 3+3 rolling mode is adopted, and the final rolling temperature of R1 is controlled at 950~1030℃ and the final rolling temperature of R2 is controlled at 980~1070℃; the cumulative reduction rate during the rough rolling stage is >70%; after rough rolling, descaling is performed, with a descaling temperature ≥1180℃ and a descaling water pressure ≥18MPa. 5) Finishing rolling: Hot rolling oil is sprayed onto the surfaces of the support rolls and work rolls simultaneously during finishing rolling; the entry temperature of finishing rolling F1 is 950~1030℃, the finishing rolling temperature of F7 is 850~900℃, the finishing rolling time is 60~100s, and descaling is performed after finishing rolling, with a descaling water pressure ≥18MPa. 6) Cooling and winding: Intermittent cooling mode is adopted. The winding temperature is set according to the product strength level: for 510L steel, the winding temperature is 580~620℃; for 610L steel and 710L steel, the winding temperature is 400~500℃.

4. The method for producing low-cost, easily formable hot-rolled automotive steel according to claim 3, characterized in that, In step 1), refined scrap steel is used for smelting. The molten steel is pretreated first, and the mass content of S in the molten steel entering the furnace is ≤0.0030%. The slag is removed. Before tapping the steel from the converter, the ladle is purged with argon gas. Slag is blocked in the early stage of tapping and slag-blocking darts are set in the later stage to ensure that the slag thickness in the ladle is <100mm.

5. The method for producing low-cost, easily formable hot-rolled automotive steel according to claim 3, characterized in that, In step 2), the tundish is purged with argon gas before pouring, and no molten steel is exposed during the pouring process. The long nozzle is cleaned after each pouring is completed, and the long nozzle is kept vertical during the pouring process. The pressure is applied in sections of 1-5mm in sections from the 5th to the 7th section of the casting machine.

6. The method for producing low-cost, easily formable hot-rolled automotive steel according to claim 3, characterized in that, In step 2), according to the measurement standard ±0.3mm, the arc connection qualification rate between the casting machine crystallizer and the casting machine zero section, between the casting machine zero section and the casting machine sector 1 section is ≥99%, between the casting machine 1 section and the casting machine 2 section, between the casting machine 7 section and the casting machine 8 section, and within each section is ≥98%.

7. The method for producing low-cost, easily formable hot-rolled automotive steel according to claim 3, characterized in that, In step 3), the heating time, heating temperature, and furnace atmosphere control for each section are as follows: Upper preheating section temperature 700–1000℃, lower preheating section temperature 700–1000℃, heating time 30–40 min; Upper first heating section temperature 1130–1170℃, lower first heating section temperature 1100–1150℃, heating time 50–60 min, using a weak oxygen atmosphere with an air-fuel ratio of 1:2.2–1:2.

4. The heating atmosphere is as follows: the upper part of the second heating section has a temperature of 1200-1250℃, the lower part of the second heating section has a temperature of 1180-1230℃, the heating time is 60-70 min, and a weak oxidizing atmosphere with an air-fuel ratio of 1:1.9-1:2.1 is used; the upper part of the soaking section has a temperature of 1180-1230℃, the lower part of the soaking section has a temperature of 1150-1200℃, the heating time is ≥35 min, and a weak oxidizing atmosphere with an air-fuel ratio of 1:2.1-1:2.5 is used.

8. The method for producing low-cost, easily formable hot-rolled automotive steel according to claim 3, characterized in that, The thickness of the intermediate billet after rough rolling is 30-80 mm.

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