A thick-gauge cold-rolled high-strength steel and a production method thereof

Abstract

A kind of thick gauge cold-rolled high-strength steel, its component and wt% amount are as follows: C: 0.15~0.25%, Mn: 0.20~0.50%, Si≤0.1%, P≤0.01%, S≤0.003%, Cr: 0.6~1.2%, Ti: 0.015~0.025%, Ca: 0.001~0.003%; production method: after molten iron desulfurization, it is conventionally smelted to set component;Slab is heated;High-pressure water descaling;Rough rolling;Finish rolling;Coiling;Pickling;Cold rolling;Annealing is carried out using full hydrogen hood furnace;Cooling to room temperature;Leveling.The present application guarantees that yield strength is in 240~360MPa, tensile strength is in 300~460MPa, makes that fracture elongation is not less than 32%, spheroidizing rate is not less than 95%, and thickness precision control difference is not more than ±30 μm, flatness control is within 7IU.

Description

Technical Field

[0001] This invention relates to a type of steel for engineering machinery and a method for producing it, specifically to a type of thick-gauge cold-rolled high-strength steel and a method for producing it. Background Technology

[0002] Cold-rolled precision stamping steel is a basic material used to manufacture high-precision parts through precision stamping processes. It is widely used in precision components in the automotive, electronics, home appliance, and machinery manufacturing industries. This type of steel requires excellent sheet quality, uniform microstructure, strict thickness tolerances, and good surface quality to ensure that complex-shaped parts can be accurately formed in one pass and have excellent mechanical properties during precision stamping.

[0003] Among existing publicly available technologies, cold-rolled precision-stamped steel, due to its high quality and performance, has relatively high production costs. This leads to increased manufacturing costs for the final product. In the highly competitive automotive and machinery manufacturing markets, cost control is crucial; therefore, a balance between material performance and cost is necessary. Furthermore, while precision-stamped steel possesses good formability, designing complex parts may still present challenges such as high forming difficulty and rapid die wear. This not only affects production efficiency but also increases scrap rates. Precise control of the material's chemical composition, microstructure, thickness accuracy, shape, and surface quality is required to ensure high precision, high finish, and good toughness during the stamping process. (As searched:)

[0004] Chinese patent application number 202111111443.4 discloses a processing technology for fine-stamped steel for friction plates, which includes the following steps: (1) hot rolling and slitting, where hot-rolled strip steel wide coils are slid into narrow strip steel using disc shears; (2) pickling, where the narrow strip steel is pickled with hydrochloric acid to remove the oxide scale on the surface of the narrow strip steel; (3) rolling, where the narrow strip steel treated in step (2) is rolled back and forth using rolls with irregular patterns on the surface until the thickness required by the user is reached; (4) stress-relief annealing, where the narrow strip steel treated in step (3) is heated to 320±5℃ and held at that temperature, and then furnace cooled to obtain fine-stamped steel with irregular patterns on the surface. This document requires the strip steel to be slid into narrow strip steel during the hot rolling process, and then further processed separately, which will affect production efficiency and production costs.

[0005] Chinese patent application No. 202211185612.3 discloses "A Cold-Rolled Fine-Blanking Steel Strip and Its Preparation Method." The chemical composition of the cold-rolled fine-blanking steel strip includes: C, Si, Mn, P, S, Al, Cr, Nb, Fe, and impurities derived from its preparation. By mass fraction, the C content is 0.80%–0.90%, and the Nb content is 0.02%–0.12%. The cold-rolled fine-blanking steel strip has a yield strength of 400–500 MPa, a tensile strength of 600–700 MPa, an elongation at break (A50) of 20–30%, and a hardness (HV5) of 165–180 HV. However, this paper uses an 18-roll cold rolling mill to produce high-hardness high-carbon steel, which results in high equipment investment and production costs.

[0006] Chinese patent application number 202010744672.9 discloses a method for manufacturing a low-alloy, low-yield-strength ratio cold-rolled steel strip for automotive shift forks. This method involves readjusting the chemical composition, microstructure, and mechanical properties of 16MnCr5 steel, a material prone to fracture during stamping, to obtain a cold-rolled, precision-stamped 16MnCr5 material that meets all the technical specifications of 16MnCr5 steel. This method requires slitting the strip into narrow strips after pickling, which also impacts production efficiency and costs. Summary of the Invention

[0007] This invention aims to overcome the shortcomings of existing technologies, such as complex processes leading to high costs, low production efficiency, poor thickness accuracy control, poor plate shape quality control, and poor microstructure uniformity control. It provides a cold-rolled high-strength steel with a thickness of 2–4 mm for precision components used in automotive and machinery manufacturing, ensuring a yield strength of 240–360 MPa, a tensile strength of 300–460 MPa, a fracture elongation of not less than 32%, a spheroidization rate of not less than 95%, a thickness accuracy control difference of no more than ±30 μm, and a plate flatness control within 7 IU.

[0008] Measures to achieve the above objectives:

[0009] A thick-gauge cold-rolled high-strength steel has the following composition and weight percentage content: C: 0.15-0.25%, Mn: 0.20-0.50%, Si≤0.1%, P≤0.01%, S≤0.003%, Cr: 0.6-1.2%, Ti: 0.015-0.025%, Ca: 0.001-0.003%, with the remainder being iron and unavoidable impurities.

[0010] Preferably, the weight percentage content of C is 0.15-0.22%.

[0011] Preferably, the weight percentage content of Mn is 0.23-0.45%.

[0012] Preferably, the weight percentage content of Si is ≤0.06%.

[0013] Preferably, the weight percentage content of Cr is 0.7% to 1.1%.

[0014] Preferably, the weight percentage content of Ti is 0.017 to 0.023%.

[0015] A method for producing a thick-gauge cold-rolled high-strength steel, comprising the following steps:

[0016] 1) After desulfurization of molten iron, it is conventionally smelted to the set composition;

[0017] 2) After being cast into slabs, the slabs are heated, and the heating temperature is controlled at 1250-1300℃, with a holding time of 130-160 minutes;

[0018] 3) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar;

[0019] 4) Perform rough rolling, and control the rough rolling temperature at 1050~1100℃;

[0020] 5) Perform finishing rolling at a final rolling temperature of 880–920℃;

[0021] 6) Perform winding, and set the winding temperature according to the thickness of the hot-rolled plate:

[0022] When the thickness of hot-rolled plate is less than 4.5 mm and 3 mm, the coiling temperature is between 576 and 584 °C.

[0023] When the thickness of the finished product is between 4.5mm and 5.5mm, the winding temperature is between 586 and 594℃.

[0024] 7) Perform pickling, controlling the acid tank temperature at 70-90℃, and control the free acid concentration in the three acid tanks:

[0025] The pickling concentration in the No. 1 pickling tank should be no less than 50 g / L, the No. 2 pickling tank should be no less than 70 g / L, and the No. 3 pickling tank should be no less than 130 g / L. The pickling speed should be controlled below 180 m / min.

[0026] 8) Cold rolling is performed using a seven-stand six-high continuous rolling mill, with the following control measures: cumulative cold rolling reduction rate between 27% and 34%; crown control of work rolls in stands 1-7: crown of work rolls in stands 1-5 between 20 and 40 μm, and crown of work roll in stand 6 less than 10 μm.

[0027] μm, the crown of the work roll of the 7th frame is 0, and a flat roll is used; and the tilt value of the 7th frame is controlled to not exceed 300μm.

[0028] The absolute value of the tension difference between the two sides of the strip at the mill exit is ≤3.5KN;

[0029] It adopts a high-precision segmented cooling spray beam device, and is used in conjunction with a high-precision segmented cooling closed-loop system and automatic control of the opening and closing of the segmented cooling nozzles;

[0030] Thickness accuracy is controlled within ±30μm, and plate flatness is controlled within 7IU;

[0031] 9) Annealing is carried out using a full hydrogen bell furnace, with the annealing temperature controlled at 680-750℃ and the holding time not less than 34 hours;

[0032] 10) Cool to room temperature;

[0033] 11) Leveling: Set the leveling elongation rate according to the thickness of the cold-rolled finished product:

[0034] When the thickness of the cold-rolled finished product is less than 2mm and less than or equal to 3mm, the flattening elongation is 1.2% to 1.4%.

[0035] When the thickness of the cold-rolled finished product is less than 3mm and less than or equal to 4mm, the flattening elongation is 1.4 to 1.6%.

[0036] Preferably, the annealing temperature is controlled at 700–730℃.

[0037] The role and mechanism of each component and main process in this invention

[0038] C: Carbon is a fundamental element in steel and the most economical and effective strengthening element. The selection of the carbon content range mainly considers the matching of strength, formability, and weldability. However, excessive carbon content reduces the plasticity and impact toughness of the steel and easily leads to center segregation, which is detrimental to bending performance and worsens cold formability and weldability. Insufficient carbon content requires the addition of a large amount of alloying to compensate for the lack of desired strength, increasing alloying costs. Based on the characteristics of automotive parts, which require high toughness and smooth welding processes during assembly, the carbon content is controlled between 0.15% and 0.25%. Preferably, the carbon content is between 0.15% and 0.22%.

[0039] Mn: Manganese is a relatively economical strengthening element that can improve the tensile strength of steel plates, moderately improve hardenability, and, in appropriate amounts, improve both toughness and cold-rolling workability. Furthermore, manganese can combine with sulfur to form manganese sulfide, significantly reducing the hot and cold brittleness caused by sulfur. Simultaneously, Mn delays the spheroidization and dissolution of cementite during annealing, inhibits cementite coarsening, refines cementite grains, and is beneficial for improving the spheroidization rate. Therefore, the Mn content is controlled at 0.2–0.5%, preferably 0.23–0.45%.

[0040] Si: A small amount of silicon can refine pearlite grains, reduce cementite network, and improve elongation. It also enhances steel purity and acts as a deoxidizer during smelting. However, increasing the silicon content significantly reduces plasticity and impact toughness, and easily forms a dense oxide layer (Mn2SiO4) on the steel plate surface, affecting the material's surface quality. Therefore, the Si content is controlled below 0.1%, preferably below 0.06%.

[0041] P and S: Both phosphorus and sulfur are harmful elements. P easily causes severe segregation, reducing the plasticity and toughness of steel plates. S easily forms sulfide inclusions in steel, which is detrimental to the material's plasticity and toughness, and also reduces corrosion resistance; therefore, sulfur content must be strictly limited. During smelting, P and S content should be minimized to improve the purity of the steel. Since further removal of P and S during smelting leads to increased smelting costs, P content is controlled below 0.01%, and S content is controlled below 0.003%.

[0042] Cr: Chromium can delay the austenite transformation incubation time, improve hardenability, postpone the transformation of ferrite and pearlite, delay the spheroidization and dissolution of cementite during annealing, inhibit cementite coarsening, refine cementite grains, and is beneficial to improving the spheroidization rate. Its effect is better than that of Mn. The amount added needs to be determined according to the carbon content and the requirements of the parts to be processed. Therefore, the Cr content is controlled at 0.6% to 1.2%, preferably 0.7% to 1.1%.

[0043] Ti: Titanium is a strong carbide and nitride-forming element, which can fix interstitial atoms C and N in steel, inhibit grain coarsening, and improve the toughness of steel. However, titanium also easily forms TiN inclusions in steel. These inclusions are relatively hard and angular, affecting the fatigue performance of the material. Therefore, through comprehensive calculations and practice, the Ti content is controlled at 0.015%–0.025%, preferably 0.017%–0.023%.

[0044] Ca: Ca treatment can control the morphology of sulfides, refine inclusions, transform elongated sulfides into spherical inclusions, reduce porosity and inclusions in molten steel, improve steel quality, and appropriately increase the steel's strength and hardness. However, excessive addition will produce CaO and CaS, forming large inclusions and impairing the steel's plasticity. Therefore, the Ca content should be controlled between 0.001% and 0.003%.

[0045] The reason why the final rolling temperature is controlled at 880-920℃ in this invention is mainly to control the amount of eutectic ferrite, pearlite and precipitated phases in the steel, as well as the amount of deformation in the non-recrystallized zone, so as to obtain suitable microstructure and mechanical properties.

[0046] The coiling temperature in this invention is set according to the thickness of the hot-rolled finished product. Specifically, when 3mm ≤ hot-rolled plate thickness < 4.5mm, the coiling temperature is 576~584℃; when 4.5mm ≤ finished product thickness ≤ 5.5mm, the coiling temperature is 586~594℃. This is to ensure the uniformity and stability of the mechanical properties of cold-rolled steel plates of different thicknesses, to refine the ferrite grains, to obtain the required strength range, and to better control the banded structure and segregation.

[0047] Due to its high hardness, cold-rolled high-strength steel often requires multiple cold rolling and annealing processes in steel production to achieve the required thickness accuracy, flatness, and microstructure. However, these multiple cold rolling and annealing processes result in high energy consumption, high costs, and environmental pollution. This invention utilizes a seven-stand, six-high continuous rolling mill for cold rolling for the following reasons: First, the seven-stand, six-high continuous rolling mill ensures good strip shape and reduces waviness; second, it allows for a wider strip width, up to 1500mm, increasing efficiency by more than three times compared to commonly used narrow strip mills (200-600mm); third, the seven-stand, six-high continuous rolling mill, through appropriate rolling processes, offers stronger rolling capacity and more uniform rolling force, allowing pickled steel strips to be rolled directly without annealing, saving several softening annealing and cold rolling passes, resulting in lower costs and greater environmental friendliness. Fourth, compared with some existing technologies such as 18-roll or 20-roll mills, the seven-stand six-roll continuous rolling mill has lower equipment investment costs, higher production efficiency, and higher product yield and surface quality.

[0048] The reason why the cumulative total reduction rate of cold rolling is controlled at 27-34% in this invention is that, on the one hand, cold-rolled high-strength steel has high hardness and excessive deformation can easily cause equipment damage, and the product shape and dimensional accuracy are difficult to control; on the other hand, it is conducive to the refinement of microstructure, and after annealing, it is easier to form more uniform and dispersed carbides, thereby improving the spheroidization rate.

[0049] This invention employs a high-precision segmented cooling system, including a spray beam device and a closed-loop control system. This system precisely controls the cooling rate of the strip during cold continuous rolling, effectively regulating the microstructure changes during the rolling process. This optimizes the material's microstructure (such as grain size and precipitate distribution), further improving product performance and surface quality. Simultaneously, the high-precision segmented cooling system can implement differentiated and refined cooling control according to different specifications and steel grades, ensuring uniform product performance and reducing fluctuations in strip shape, thickness, and performance differences caused by uneven cooling.

[0050] The present invention controls the crown of the work rolls of stands 1-7, specifically, the crown of the work rolls of stands 1-5 is 20-40 μm, the crown of the work roll of stand 6 is less than 10 μm, and the crown of the work roll of stand 7 is 0 μm; and uses flat rolls, while controlling the inclination value of stand 7 to below 200 μm, to ensure that the absolute value of the tension difference between the two sides of the strip at the mill exit is ≤3.5 kN. This reduces the tension difference between the two sides of the strip in the width direction, effectively improving the strip shape quality and ensuring that the flatness of the product is controlled within 7 IU.

[0051] This invention employs a seven-stand AGC (Automatic Gauge Control) model. Based on the thickness gauge readings, the AGC model automatically calculates compensation values ​​after comparing them to the target thickness. By controlling the hydraulic pressing and main drive speed of the rolling mill, it ensures the strip thickness reaches the target value, guaranteeing product thickness accuracy within ±30μm.

[0052] The reason this invention controls the annealing temperature in a hydrogen-filled bell-type furnace to be between 680 and 750°C, preferably between 700 and 730°C, with a holding time of no less than 34 hours, is that the annealing temperature needs to be controlled above the recrystallization temperature and below the austenitizing temperature. This allows for complete recrystallization of the microstructure while preventing excessive grain growth. However, if the annealing temperature is too high, it can easily lead to coarse ferrite grains, increased softening of the matrix, and decreased strength and hardness. Simultaneously, free carbides aggregate and grow, damaging the plasticity of the steel. Therefore, the annealing temperature range is between 680 and 750°C, preferably between 700 and 730°C. After selecting a suitable annealing temperature, an appropriate holding time should be chosen to achieve the best match between the annealing temperature and the holding time. The annealing time needs to ensure that the strip steel structure has sufficient time to recover and recrystallize. If the holding time is too short, the fibrous structure elongated after cold rolling will not have enough time to recrystallize, or only a small portion of the grains will have completed recrystallization, affecting the plasticity of the steel. If the holding time is too long, the recrystallized grains will become excessively coarse, and the cementite will gradually grow, affecting the formability of the steel. Therefore, the holding time should be controlled to be no less than 34 hours.

[0053] The reason why this invention adopts the method of setting the flattening elongation rate according to the thickness of the cold-rolled finished product, that is, when 2mm < cold-rolled finished product thickness ≤ 3mm, the flattening elongation rate is 1.2~1.4%; when 3mm < cold-rolled finished product thickness ≤ 4mm, the flattening elongation rate is 1.4~1.6%, is to ensure the uniformity and stability of the mechanical properties of cold-rolled steel plates of different thicknesses, while giving the steel plates good surface quality and roughness.

[0054] Compared with the prior art, the present invention ensures that the yield strength is between 240 and 360 MPa, the tensile strength is between 300 and 460 MPa, the elongation at break is not less than 32%, the spheroidization rate is not less than 95%, the thickness accuracy control difference is not more than ±30 μm, and the plate flatness is controlled within 7 IU. Attached Figure Description

[0055] Figure 1 This is an image of the metallographic structure of the present invention, showing a uniformly distributed spheroidized structure and ferrite at 500x magnification, wherein the spheroidization rate is ≥95%. Detailed Implementation

[0056] The present invention will now be described in detail:

[0057] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0058] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0059] Table 3 is a list of performance test results for each embodiment and comparative example of the present invention.

[0060] The various embodiments of the present invention are produced according to the following steps.

[0061] 1) After desulfurization of molten iron, it is conventionally smelted to the set composition;

[0062] 2) After being cast into slabs, the slabs are heated, and the heating temperature is controlled at 1250-1300℃, with a holding time of 130-160 minutes;

[0063] 3) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar;

[0064] 4) Perform rough rolling, and control the rough rolling temperature at 1050~1100℃;

[0065] 5) Perform finishing rolling at a final rolling temperature of 880–920℃;

[0066] 6) Perform winding, and set the winding temperature according to the thickness of the hot-rolled plate:

[0067] When the thickness of hot-rolled plate is less than 4.5 mm and 3 mm, the coiling temperature is between 576 and 584 °C.

[0068] When the thickness of the finished product is between 4.5mm and 5.5mm, the winding temperature is between 586 and 594℃.

[0069] 7) Perform pickling, controlling the acid tank temperature at 70-90℃, and control the free acid concentration in the three acid tanks:

[0070] The pickling concentration in the No. 1 pickling tank should be no less than 50 g / L, the No. 2 pickling tank should be no less than 70 g / L, and the No. 3 pickling tank should be no less than 130 g / L. The pickling speed should be controlled below 180 m / min.

[0071] 8) Cold rolling is performed using a seven-stand six-high continuous rolling mill, with the following control measures: cumulative cold rolling reduction rate between 27% and 34%; crown control of work rolls in stands 1-7: crown of work rolls in stands 1-5 between 20 and 40 μm, and crown of work roll in stand 6 less than 10 μm.

[0072] μm, the crown of the work roll of the 7th frame is 0, and a flat roll is used; and the tilt value of the 7th frame is controlled to not exceed 300μm.

[0073] The absolute value of the tension difference between the two sides of the strip at the mill exit is ≤3.5KN;

[0074] It adopts a high-precision segmented cooling spray beam device, and is used in conjunction with a high-precision segmented cooling closed-loop system and automatic control of the opening and closing of the segmented cooling nozzles;

[0075] Thickness accuracy is controlled within ±30μm, and plate flatness is controlled within 7IU;

[0076] 9) Annealing is carried out using a full hydrogen bell furnace, with the annealing temperature controlled at 680-750℃ and the holding time not less than 34 hours;

[0077] 10) Cool to room temperature;

[0078] 11) Leveling: Set the leveling elongation rate according to the thickness of the cold-rolled finished product:

[0079] When the thickness of the cold-rolled finished product is less than 2mm and less than or equal to 3mm, the flattening elongation is 1.2% to 1.4%.

[0080] When the thickness of the cold-rolled finished product is less than 3mm and less than or equal to 4mm, the flattening elongation is 1.4 to 1.6%.

[0081] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0082]

[0083] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0084]

[0085] Continued from Table 2

[0086]

[0087] Table 3. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0088]

[0089] Note: The tensile test specimens for the mechanical properties of this invention are longitudinal specimens with L0 = 50 mm.

[0090] As can be seen from Table 3, the mechanical properties, spheroidization rate, thickness accuracy, and flatness of the plate of the present invention all meet the target. As can be seen from the comparative examples, Comparative Example 1 has excessive strip tension difference at the mill exit, resulting in the final flatness of the product exceeding the standard, reaching 18 IU, and the thickness accuracy exceeding the standard, reaching -68 μm. Comparative Example 2 has excessive mechanical properties due to low annealing temperature and short holding time. The microstructure is a dotted spheroidized structure + a small amount of pearlite + ferrite, and the spheroidization rate is below 70%.

[0091] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A method for producing a thick-gauge cold-rolled high-strength steel, comprising the following steps: 1) After desulfurization of molten iron, it is conventionally smelted to the set composition; 2) After being cast into slabs, the slabs are heated, and the heating temperature is controlled at 1269-1300℃, with a holding time of 130-160 minutes; 3) Perform high-pressure water descaling before rolling, and control the descaling water pressure between 280 and 420 bar; 4) Perform rough rolling, and control the rough rolling temperature at 1068~1100℃; 5) Perform finishing rolling at a final rolling temperature of 880–920℃; 6) Perform winding and set the winding temperature according to the thickness of the hot-rolled plate: When the thickness of hot-rolled plate is less than 4.5 mm and 3 mm, the coiling temperature is between 576 and 584 °C. When the thickness of the finished product is between 4.5mm and 5.5mm, the winding temperature is between 586 and 594℃. 7) Perform pickling, controlling the acid tank temperature at 70-90℃, and controlling the free acid concentration in the three acid tanks: Pickling rate: No. 1 pickling tank: no less than 50 g / L; No. 2 pickling tank: no less than 70 g / L; No. 3 pickling tank: no less than 130 g / L. Keep it below 180 m / min; 8) Cold rolling is carried out using a seven-stand six-high continuous rolling mill, with the following controls: cumulative cold rolling reduction rate of 27-29%; crown control of work rolls of stands 1-7: crown of work rolls of stands 1-5 is 20-40μm, crown of work roll of stand 6 is less than 10μm, crown of work roll of stand 7 is 0, and flat rolls are used; and the tilt value of stand 7 is controlled not to exceed 300μm, and the absolute value of the tension difference between the two sides of the strip at the mill exit is ≤3.5KN; It adopts a high-precision segmented cooling spray beam device, and is used in conjunction with a high-precision segmented cooling closed-loop system and automatic control of the opening and closing of the segmented cooling nozzles; Thickness accuracy is controlled within ±30μm, and plate flatness is controlled within 7IU; 9) Annealing is carried out using a full hydrogen bell furnace, with the annealing temperature controlled at 680-750℃ and the holding time not less than 34 hours; 10) Cool to room temperature; 11) Leveling: Set the leveling elongation rate according to the thickness of the cold-rolled finished product; The thick-gauge cold-rolled high-strength steel has the following composition and weight percentage content: C: 0.19-0.25%, Mn: 0.20-0.44%, Si: 0.06-0.07%, P≤0.01%, S≤0.003%, Cr: 0.6-1.2%, Ti: 0.015-0.025%, Ca: 0.001-0.003%, with the remainder being iron and unavoidable impurities.

2. The method for producing a thick-gauge cold-rolled high-strength steel as described in claim 1, characterized in that: The annealing temperature is controlled at 700-730℃.

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