1.5-3.1 mm high-ductility and high-toughness pickling automobile steel and manufacturing method thereof

Abstract

The invention relates to 1.5-3.1 mm high-plasticity toughness acid pickling automobile steel and a manufacturing method thereof. The steel plate comprises the following chemical components in percentage by weight: 0.045 to 0.065 weight percent of C, less than or equal to 0.30 weight percent of Si, 1.15 to 1.75 weight percent of Mn, 0.15 to 0.55 weight percent of Als, 0.015 to 0.050 weight percent of Nb, 0.010 to 0.065 weight percent of Ti, 0.0005 to 0.0031 weight percent of Ca, less than or equal to 0.023 weight percent of P, less than or equal to 0.0050 weight percent of S, less than or equal to 0.0050 weight percent of N and the balance of Fe and inevitable impurities. The manufacturing technological process of the pickling automobile steel comprises the steps of molten iron pretreatment, converter smelting, alloy fine adjustment, LF furnace refining, continuous casting, hot rolling (-finishing), pickling (-leveling), oil coating and fine packaging and pickling of coils, continuous casting blanks are subjected to corner cleaning-free and widening-free hot charging rolling, the surface quality grade of the pickling coils is FC, the yield strength in the 90-degree direction is 510-590 MPa, the tensile strength is 580-665 MPa, and the yield strength in the 90-degree direction is 510-590 MPa. The elongation A50 is larger than or equal to 21.5%, the hole expansion rate (HER) is larger than or equal to 80%, and the material is excellent in stamping forming performance at market clients, high in customer satisfaction and high in market competitiveness.

Description

Technical Field

[0001] This invention relates to the field of pickled automotive steel manufacturing technology, and more particularly to a 1.5~3.1mm pickled automotive steel and its manufacturing method. Technical Background Compared to hot-dip galvanized and continuously annealed automotive steel of the same thickness, pickled automotive steel has relatively lower n-values ​​and r-values, resulting in a lack of significant competitive advantage in formability. Furthermore, compared to hot-dip galvanized automotive steel, pickled automotive steel exhibits poorer corrosion resistance. Therefore, while maintaining a price advantage, pickled automotive steel should strive to maximize its formability to prevent any forming failures during market application, thereby increasing customer satisfaction and securing long-term, stable orders.

[0002] Existing technologies at home and abroad show that in order to ensure that pickled automotive steel with a tensile strength of 600MPa has excellent formability in the market, high-Si alloy design is generally adopted. However, Si is one of the key factors affecting the surface quality of pickled coils. In particular, when the key process control of hot rolling, such as the continuous casting billet exit temperature, furnace time, and rough rolling descaling, does not meet the technical standards, the high surface quality and stability of pickled coils cannot be guaranteed.

[0003] Chinese invention patent CN114540707B discloses "a 590MPa grade cold-rolled high-strength steel and its production method". The chemical composition of the steel plate by weight percentage is: C 0.07~0.14 wt.%, Si 0.40~0.70 wt.%, Mn 1.50~2.00 wt.%, Al 0.25~0.50 wt.%, P≤0.012 wt.%, S≤0.008 wt.%, N≤0.004 wt.%, containing at least one element Ti and Nb, Ti 0.005~0.015 wt.%, Nb 0.005~0.015 wt.%, and the Si and Al content must also meet the requirements of Si+Al≥0.60 and 1.0≤Si / Al≤1.6, with the balance being Fe and unavoidable impurities. The manufacturing process of the steel plate involved is as follows: smelting → casting → slab heating → hot rolling → cooling → coiling → pickling → cold rolling → continuous annealing. The metallographic structure of the steel plate contains 65-80% ferrite, 10-20% bainite, 1-15% martensite, and 3-8% retained austenite by volume, with a ferrite grain size ≤5µm. The yield strength of the steel plate is ≥340MPa, the tensile strength is ≥590MPa, and the elongation is A... 80≥30%, uniform elongation ≥16%, expansion rate (HER) ≥40%. The invention patent product and process have the following characteristics: (1) The heating time of the slab in the hot rolling process is 30~100min. Due to the short heating time, the rolling difficulty will increase. (2) The average C element content is relatively high, and the equivalent carbon equivalent is greater than 0.080, which makes it impossible to achieve continuous casting slab without cleaning the corner and without widening the hot charging rolling. (3) The average Si element content is relatively high, and there are obvious red rust defects on the surface of the hot rolled coil.

[0004] Chinese invention patent CN113186461B discloses "a high-strength, high-ductility, cryogenically rolled steel plate and its preparation method." The chemical composition of the steel plate by weight percentage is: C 0.23~0.25 wt.%, Si 0.60~1.50 wt.%, Mn 2.80~3.50%, Al 0.50~0.90 wt.%, V 0.13~0.15 wt.%, Cr 0.30~0.50 wt.%, Ni 0.10~0.50 wt.%, Mo 0.10~0.50 wt.%, Cu 0.10~0.50 wt.%, Nb 0.020~0.030 wt.%, B 0.002~0.005 wt.%, Ti≤0.005 wt.%, Ca≤0.005 wt.%, P≤0.050 wt.%, S≤0.050 wt. wt.%, and Si+Al≤2.00 wt.%, the balance is Fe and unavoidable impurities. The manufacturing process of the steel plate involved is: smelting → continuous casting → hot rolling → deep cold rolling → continuous annealing. The metallographic structure of the steel plate contains a certain amount of reversed austenite, the yield strength is 650~800MPa, the tensile strength is ≥1000MPa, the elongation is ≥30%, and the strength-ductility product is ≥30GPa%. The product and process of this invention patent have the following characteristics: (1) The alloy content of the patent product is significantly higher, and the welding difficulty is extremely high. (2) The alloy cost is significantly higher, which directly affects its market competitiveness. (3) Under the premise that the Si element content is significantly higher, it is not explained what method is used to eliminate its influence on the surface quality of the pickled coil.

[0005] Chinese invention patent CN111363901B discloses "a hot-rolled ferritic martensitic dual-phase steel with high surface quality and its manufacturing method". The chemical composition of the steel plate by weight percentage is: C 0.04~0.08 wt.%, Si≤0.10 wt.%, Mn 1.00~1.40 wt.%, Al 0.30~0.80 wt.%, Cr 0.15~0.60 wt.%, Nb 0.01~0.03 wt.%, P≤0.015wt.%, S≤0.002 wt.%, N≤0.005 wt.%, containing at least one of Ti, V and Ca elements, with Ti≤0.030 wt.%, V≤0.030 wt.%, Ca≤0.005 wt.%, and also satisfying Si+2Al+Cr≥1.0 and Al / Cr≤2.5, with the balance being Fe and unavoidable impurities. This patented product contains Cr (chromium), and when the Cr content exceeds 0.45%, it will directly affect the surface quality of the continuously cast billet. The porosity (HER) of the steel plate involved in this patented invention is <80%.

[0006] Existing publicly available technologies cannot simultaneously possess the advantages of continuous casting billets that require no corner clearing and no widening during hot-charging rolling, high plasticity and toughness, high surface quality, and low cost. This invention aims to provide a 1.5~3.1mm high plasticity and toughness pickled automotive steel and its manufacturing method. The continuous casting billet is produced without corner clearing and no widening during hot-charging rolling, resulting in low overall cost. The material exhibits excellent stamping and forming performance and superior surface quality, meeting the high-performance order supply needs of market customers. Summary of the Invention

[0007] To achieve the above-mentioned technical effects, this invention provides a 1.5~3.1mm high-ductility pickled automotive steel and its manufacturing method. The continuously cast billet is hot-rolled without corner cleaning or widening. The surface quality grade of the pickled coil is FC. The yield strength in the 90° direction is 510~590MPa, the tensile strength is 580~665MPa, the elongation A50 is ≥21.5%, and the expansion ratio (HER) is ≥80%. The material has excellent stamping performance, excellent surface quality, high customer satisfaction, and strong market competitiveness. It can replace hot-dip galvanized automotive steel and continuously annealed automotive steel of the same thickness for the manufacture and application of automotive structural parts.

[0008] The present invention discloses a 1.5~3.1mm high-ductility pickled automotive steel, characterized in that the chemical composition of the steel plate by weight percentage is: C 0.045~0.065 wt.%, Si≤0.30 wt.%, Mn 1.15~1.75 wt.%, Als 0.15~0.55 wt.%, Nb 0.015~0.050 wt.%, Ti 0.010~0.065 wt.%, Ca 0.0005~0.0031 wt.%, P≤0.023 wt.%, S≤0.0050 wt.%, N≤0.0050 wt.%, with the balance being Fe and unavoidable impurities.

[0009] Furthermore, the chemical composition of the steel plate by weight percentage is as follows: C 0.045~0.060 wt.%, Si≤0.25 wt.%, Mn1.35~1.65 wt.%, Als 0.19~0.50 wt.%, Nb 0.023~0.045 wt.%, Ti 0.015~0.055 wt.%, Ca0.0010~0.0025 wt.%, P≤0.015 wt.%, S≤0.0039 wt.%, N≤0.0045 wt.%, with the balance being Fe and unavoidable impurities.

[0010] Furthermore, the width of the pickled automotive steel is 900~1680mm.

[0011] Furthermore, the microstructure of the pickled automotive steel consists of 71-83.5% ferrite by volume and 16.5-29% granular bainite by volume, with the ferrite grain size being 11-13.5.

[0012] Furthermore, the pickled automotive steel has a yield strength of 510~590MPa in the 90° direction, a tensile strength of 580~665MPa, and an elongation A. 50 ≥21.5%, expansion ratio (HER) ≥80%.

[0013] Furthermore, the manufacturing process of the pickled automotive steel is as follows: molten iron pretreatment → converter smelting → alloy fine-tuning → LF furnace refining → continuous casting → hot rolling (→ finishing) → pickling (→ leveling) → oiling and fine packaging → pickled coil.

[0014] A method for manufacturing the above-mentioned 1.5~3.1mm high-ductility and high-toughness pickled automotive steel is characterized by comprising the following steps: S1. Steelmaking and continuous casting process: After the LF furnace refining process is completed, calcium treatment is carried out, and argon blowing with stopper rods promotes the flotation of inclusions. The temperature of the tundish is controlled at 10~30℃ above the target liquidus temperature. Dynamic light reduction and electromagnetic rollers are put into use. The light reduction is 4.5~6.5mm, the rated current of the electromagnetic roller is 135~160A, and the rated frequency is 2.3~3.9HZ. Smelting and continuous casting billet are produced. The continuous casting billet adopts hot charging without cleaning corners or widening. S2. Hot rolling process: The furnace exit temperature of the continuous casting billet at any position in the rolling direction is 1215~1265℃, the furnace time of the continuous casting billet is 160~250min, the final rolling temperature is 830~890℃, and the length of the laminar flow cooling section is not less than 65m. S3. Pickling process: The acid solution temperature is 65~86℃, the flattening elongation is 0.3~0.9%, and the pickling line operating speed does not exceed 190m / min; the strip steel is dried at a temperature range of 100~150℃, and then electrostatically oiled, with an oiling amount of 1.0~3.1g / m. 2 After fine packaging, the acid-washed rolls are obtained.

[0015] Further, in step S1, the casting speed of the continuous casting billet is controlled within the range of 1.0~1.5m / min, and the thickness of the continuous casting billet is 190~250mm.

[0016] Further, in step S2, the laminar flow cooling method of the hot rolling process adopts discontinuous cooling. First, the strip steel is water-cooled to a temperature range of 650~695℃, then air-cooled for 3~9 seconds, and then water-cooled to a temperature range of 410~500℃ for coiling to obtain hot-rolled coil. If the hot-rolled coil has poor shape, finishing is required to correct the shape. The finishing elongation rate is 0.5~2.3%. If the hot-rolled coil has good shape, finishing is not required (it can be directly put into the pickling line).

[0017] Further, in step S3, the acid concentration in pickling tank #1 is 31~68g / L, the acid concentration in tank #2 is 95~150g / L, and the acid concentration in tank #3 is 135~190g / L.

[0018] The beneficial effects achieved by this invention are as follows: 1. The steel zone adopts a high-Al alloy design to purify ferrite and improve the toughness of pickled coils; 2. The laminar flow cooling section of the rolling zone adopts a discontinuous cooling method to increase the proportion of ferrite (soft phase); 3. Under low-temperature winding process conditions, Ti microalloying is achieved, grains are refined, and pickling, winding, flanging, and hole expansion forming performance is excellent. Attached Figure Description

[0019] Figure 1 The image shows the metallographic structure of 1.8~2.3mm thick high-ductility pickled automotive steel from Example 1. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the technical solution of the present invention and its technical inspiration, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The chemical composition (by weight percentage) of the 1.5~3.1mm high-ductility pickled automotive steel sheet described in this invention is as follows: C 0.045~0.065 wt.%, Si≤0.30 wt.%, Mn 1.15~1.75 wt.%, Als 0.15~0.55 wt.%, Nb 0.015~0.050 wt.%, Ti 0.010~0.065 wt.%, Ca 0.0005~0.0031 wt.%, P≤0.023 wt.%, S≤0.0050 wt.%, N≤0.0050 wt.%, with the balance being Fe and unavoidable impurities.

[0022] Furthermore, the chemical composition of the steel plate by weight percentage is as follows: C 0.045~0.0060 wt.%, Si≤0.25 wt.%, Mn 1.35~1.65 wt.%, Als 0.19~0.50 wt.%, Nb 0.023~0.045 wt.%, Ti 0.015~0.055 wt.%, Ca 0.0010~0.0031 wt.%, P≤0.015 wt.%, S≤0.0039 wt.%, N≤0.0045 wt.%, with the balance being Fe and unavoidable impurities.

[0023] The role of each alloying element and the reasons for controlling its content in the 1.5~3.1mm high-ductility pickling automotive steel described in this invention are as follows: C: C element plays a solid solution strengthening role, but its content is inversely proportional to the plasticity and toughness of steel. In this invention, the C element content is controlled at 0.045~0.065 wt.%.

[0024] Si: Si elements refine ferrite and improve the uniformity of the microstructure, but excessive Si content will significantly reduce the surface quality of pickled rolls. In this invention, the Si element content is controlled below 0.30 wt.%.

[0025] Mn: Mn plays a role in solid solution strengthening and is inexpensive, but excessive Mn content can easily cause segregation and reduce the uniformity of the structure. In this invention, the Mn content is controlled at 1.15~1.75 wt.%.

[0026] Al: Al plays a role in purifying ferrite and is beneficial to improving the toughness of pickled coils. However, excessively high Al content will increase the difficulty of controlling inclusions in the steel area. Therefore, this invention controls the Al content at 0.15~0.55 wt.%.

[0027] Nb: Nb is an excellent element for improving strength without reducing ductility and toughness, but its price is relatively high. In this invention, the Nb content is controlled at 0.015~0.050%.

[0028] Ti: Ti element plays a role in grain refinement and precipitation strengthening, but it is prone to forming inclusions in the steel zone and is highly susceptible to the effects of hot rolling process in the rolling zone. Taking all factors into consideration, the present invention controls the Ti element content at 0.010~0.065 wt.%.

[0029] Ca: Ca plays a role in deoxidation, desulfurization and improving non-metallic inclusions, but if the content is too high, it will reduce the purity of the molten steel. In this invention, the Ca content is controlled at 0.0005~0.0031%.

[0030] P: P element tends to segregate at grain boundaries. In this invention, the P element content is controlled to be below 0.023%.

[0031] S: S is a harmful impurity element that directly reduces the plasticity and toughness of steel. This invention controls the S content to below 0.0050%.

[0032] N: Excessive N content will directly affect the amount of Ti-containing precipitates and reduce the strength of pickled rolls. Therefore, the present invention controls the N content to below 0.0050%.

[0033] This invention employs a high-Al alloy design in the steel zone to purify ferrite and improve the toughness of pickled coils. The laminar cooling section of the rolling zone utilizes a discontinuous cooling method to increase the proportion of ferrite (soft phase). Under low-temperature coiling conditions, Ti microalloying is achieved, refining the grains and resulting in excellent edge-flanging and hole-expanding forming performance of the pickled coils.

[0034] The manufacturing process of the 1.5~3.1mm high plasticity and toughness pickled automotive steel is as follows: molten iron pretreatment → converter smelting → alloy fine adjustment → LF furnace refining → continuous casting → hot rolling (→ finishing) → pickling (→ leveling) → oiling and fine packaging → pickled coil.

[0035] like Figure 1 As shown, specific embodiments will be used to further illustrate the technical solution of the present invention.

[0036] After hot metal pretreatment, converter smelting, alloy fine-tuning, LF furnace refining, and continuous casting, high-ductility and toughness pickled automotive steel continuous casting billets with alloy element content of 1.5~3.1mm were manufactured, as shown in Table 1 (Examples 1~5).

[0037] Table 1. Alloy element content of the pickled automotive steel continuous casting billet described in this invention. (2) The continuously cast billet is heated, descaled, rough rolled, finish rolled, laminar flow cooled and coiled to obtain hot-rolled coil. The key process parameters of the hot rolling process are shown in Table 2. If the hot-rolled coil has a poor shape, finishing is required to correct the shape. The finishing elongation rate is 0.5~2.3%. If the hot-rolled coil has a good shape, finishing is not required.

[0038] Table 2 Key process parameters of the hot rolling process for pickling automotive steel described in this invention (3) The hot-rolled coils are pickled (leveled), oiled, and finely packaged to obtain pickled coils. The mechanical properties, application properties, and surface quality of the pickled coils in Examples 1-5 are shown in Table 3.

[0039] Table 3 Mechanical properties, application properties, and surface quality of the pickled automotive steel described in this invention. Note: The mechanical property testing methods for the pickled rolls in Examples 1-5 refer to the national standard GB / T 228.1-2010, test type number P6, and the sample direction is transverse (90° direction). The elongation A is automatically measured using an extensometer. 50 (Without using dot measurement, dot measurement elongation A) 50 The numerical value will show a certain degree of increase. The method for determining porosity (HER) strictly follows the national standard GB / T15825.4-2008.

[0040] Experimental data show that the 1.5~3.1mm high-ductility and high-toughness pickled automotive steel described in this invention, produced by hot-rolling continuously cast billets without corner cleaning or widening, has a surface quality grade of FC. Its microstructure consists of 71~83.5% ferrite and 16.5~29% granular bainite by volume, with a ferrite grain size of 11~13.5. The yield strength in the 90° direction is 510~590MPa, the tensile strength is 580~665MPa, and the elongation is A... 50 With a porosity of ≥21.5% and a hole expansion ratio (HER) of ≥80%, the material exhibits excellent stamping performance in the market, resulting in high customer satisfaction and strong market competitiveness.

[0041] It should be noted that the description of the above technical solutions is exemplary, and this specification may be embodied in different forms and should not be construed as limiting it to the technical solutions set forth herein. Rather, providing these descriptions will ensure that the disclosure of this invention is thorough and complete, and will fully convey the scope of the technology disclosed herein to those skilled in the art. Furthermore, the technical solutions of this invention are defined only by the scope of the claims.

[0042] It is worth emphasizing that the above embodiments are merely representative illustrative examples of the present invention. Obviously, the present invention is not limited to the above embodiments, and many technical modifications are possible. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention should be considered within the protection scope of the present invention.

Claims

1. A 1.5~3.1mm high-ductility and high-toughness pickled automotive steel, characterized in that: The chemical composition of the steel plate by weight percentage is: C 0.045~0.065 wt.%, Si≤0.30 wt.%, Mn 1.15~1.75 wt.%, Als 0.15~0.55 wt.%, Nb 0.015~0.050 wt.%, Ti 0.010~0.065 wt.%, Ca 0.0005~0.0031 wt.%, P≤0.023 wt.%, S≤0.0050 wt.%, N≤0.0050 wt.%, with the balance being Fe and unavoidable impurities; the metallographic structure of the steel plate is 71~83.5% ferrite by volume and 16.5~29% granular bainite by volume, with a ferrite grain size of 11~13.

5.

2. The 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 1, characterized in that, The chemical composition of the steel plate by weight percentage is as follows: C 0.045~0.060 wt.%, Si≤0.25 wt.%, Mn 1.35~1.65 wt.%, Als 0.19~0.50 wt.%, Nb 0.023~0.045 wt.%, Ti 0.015~0.055 wt.%, Ca 0.0010~0.0025 wt.%, P≤0.015 wt.%, S≤0.0039 wt.%, N≤0.0045 wt.%, with the balance being Fe and unavoidable impurities.

3. The 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 1, characterized in that, The width of pickled automotive steel ranges from 900 to 1680 mm.

4. The 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 1, characterized in that, The yield strength in the 90° direction is 510~590MPa, the tensile strength is 580~665MPa, the elongation is ≥21.5%, and the porosity is ≥80%.

5. The 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 1, characterized in that, Its manufacturing process is as follows: molten iron pretreatment → converter smelting → alloy fine-tuning → LF furnace refining → continuous casting → hot rolling → finishing → pickling → leveling → oiling and packaging → pickled coil.

6. A method for manufacturing 1.5-3.1mm high-ductility and high-toughness pickled automotive steel as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Steelmaking and continuous casting process: After the LF furnace refining process is completed, calcium treatment is carried out, and argon blowing with stopper rods promotes the flotation of inclusions. The temperature of the tundish is controlled at 10~30℃ above the target liquidus temperature. Dynamic light reduction and electromagnetic rollers are put into use. The light reduction is 4.5~6.5mm, the rated current of the electromagnetic roller is 135~160A, and the rated frequency is 2.3~3.9HZ. Smelting and continuous casting billet are produced. The continuous casting billet adopts hot charging without cleaning corners or widening. S2. Hot rolling process: The furnace exit temperature of the continuous casting billet at any position in the rolling direction is 1215~1265℃, the furnace time of the continuous casting billet is 160~250min, the final rolling temperature is 830~890℃, and the length of the laminar flow cooling section is not less than 65m. S3. Pickling process: The acid solution temperature is 65~86℃, the flattening elongation is 0.3~0.9%, and the pickling line operating speed does not exceed 190m / min; the strip steel is dried at a temperature range of 100~150℃, and then electrostatically oiled, with an oiling amount of 1.0~3.1g / m. 2 After packaging, pickled rolls are obtained.

7. The method for manufacturing 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 6, characterized in that, In step S1, the casting speed of the continuous casting billet is controlled within the range of 1.0~1.5m / min, and the thickness of the continuous casting billet is 190~250mm.

8. The method for manufacturing 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 6, characterized in that, In step S2, the laminar flow cooling section of the hot rolling process adopts a discontinuous cooling method. First, the strip steel is water-cooled to a temperature range of 650~695℃, then air-cooled for 3~9 seconds, and then water-cooled to a temperature range of 410~500℃ for coiling to obtain a hot-rolled coil. If the hot-rolled coil has poor shape, it needs to be finished to correct the shape. The finishing elongation rate is 0.5~2.3%. If the hot-rolled coil has good shape, it does not need to be finished and is directly sent to the pickling line.

9. The method for manufacturing 1.5~3.1mm high-ductility and high-toughness pickled automotive steel according to claim 6, characterized in that, In step S3, the acid concentration in pickling tank #1 is 31~68g / L, the acid concentration in tank #2 is 95~150g / L, and the acid concentration in tank #3 is 135~190g / L.

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