Production method of cold-rolled ultralow-carbon stamping mild steel based on cover annealing and product

Abstract

The invention provides a cover annealing-based cold-rolled ultralow-carbon stamping mild steel production method and a product, the production method comprises the following steps: continuous casting is carried out to obtain a casting blank, and the casting blank comprises the following components in percentage by mass: less than or equal to 0.005% of C, 0-0.05% of Si, 0-0.3% of Mn, 0-0.02% of P, 0-0.018% of S, 0.02-0.06% of Alt, 0-0.005% of N, and the balance of Fe and inevitable inclusions; the casting blank is heated to 1180-1250 DEG C, hot rolling is conducted in a single-phase austenite area, a finish rolling steel plate is obtained, and the finish rolling temperature of hot rolling is 870-920 DEG C; coiling and cold rolling the hot-rolled steel plate to obtain a cold-rolled steel plate; the cold-rolled steel plate is subjected to cover annealing, the cover annealing temperature is 670-710 DEG C, and an annealed steel plate is obtained; and the annealed steel plate is flattened, and the cold-rolled ultralow-carbon stamping mild steel is obtained. According to the production method of the cold-rolled ultralow-carbon stamping mild steel, the excellent forming performance and low yield strength can be achieved, and meanwhile the cost is lower.

Description

TECHNICAL FIELD

[0001] The application relates to the alloy steel technology field, in particular to a production method of cold-rolled ultra-low-carbon stamping soft steel based on cover annealing and a product. BACKGROUND

[0002] The cold-rolled stamping soft steel is the most common steel material, and is widely applied to the automobile, household appliance, hardware, building and other industries, such as automobile parts, household appliance shell, cable bridge, oil drum, furniture hinge and anti-theft door, and is closely related to people's production and life.

[0003] The low-carbon stamping soft steel has relatively high strength and relatively general plastic forming performance, and defects such as cracking and wrinkling occur when the low-carbon stamping soft steel is applied to stamping parts with a larger stamping deformation, so that the use requirement cannot be met. SUMMARY

[0004] The application provides a production method of cold-rolled ultra-low-carbon stamping soft steel based on cover annealing and a product, and excellent forming performance and low yield strength can be realized, and the cost is lower.

[0005] In a first aspect, the application provides a production method of cold-rolled ultra-low-carbon stamping soft steel based on cover annealing, which comprises the following steps: continuous casting to obtain a casting blank, the casting blank comprising the following components in mass percentage: C: ≤0.005%, Si: 0-0.05%, Mn: 0-0.3%, P: 0-0.02%, S: 0-0.018%, Alt: 0.02-0.06%, N: 0-0.005%, and the balance being Fe and unavoidable inclusions; heating the casting blank to 1180-1250 DEG C to perform finish rolling in a single-phase austenite zone to obtain a finish-rolled steel plate, and the finish rolling has a finish rolling temperature of 870-920 DEG C; coiling the finish-rolled steel plate to obtain a steel coil, and the coiling has a coiling temperature of 640-730 DEG C and a coiling speed of 1.2-2.0 m / s; cold-rolling the steel coil to obtain a cold-rolled steel plate, and controlling the cumulative reduction rate to be 60-80%; performing cover annealing on the cold-rolled steel plate to obtain an annealed steel plate, and the cover annealing has a temperature of 670-710 DEG C and a holding time of 5-10 h; and performing skin pass on the annealed steel plate to obtain the cold-rolled ultra-low-carbon stamping soft steel.

[0006] According to the embodiment of the first aspect of the application, the finish rolling process has 7 passes, the cumulative reduction rate of the first 3 passes is 40-50%, the reduction rate of the last 4 passes is gradually reduced, and an edge heating device is used after the fourth pass to make the temperature difference between the edge and the center of the casting blank ≤15 DEG C, the water temperature at the finish rolling inlet ≤30 DEG C, the water temperature at the finish rolling outlet ≤45 DEG C, and the cooling time between adjacent passes is 3-5 s.

[0007] According to the embodiment of the first aspect of the present application, after finish rolling, the finish-rolled steel plate is further subjected to laminar cooling, the laminar cooling adopts a cooling mode of partitioned control along the running direction of the finish-rolled steel plate, including a front cooling zone and a rear cooling zone; wherein the water spraying density of the front cooling zone is controlled to be 10-15 L / (m2·s), and the water spraying density of the rear cooling zone is controlled to be 3-5 L / (m2·s).

[0008] According to the embodiment of the first aspect of the present application, the cover annealing is full-hydrogen cover furnace annealing, and the working parameters include: a temperature rising rate of 50-80 ℃ / h from room temperature to 400 ℃, a temperature rising rate of 30-50 ℃ / h from 400 ℃ to the annealing temperature, and a hydrogen flow rate of 50-80 m3 / h in the holding stage.

[0009] According to the embodiment of the first aspect of the present application, after the holding ends, the cold-rolled steel plate is cooled to below 180 ℃ out of the furnace, and the cooling rate is ≤28 ℃ / h.

[0010] According to the embodiment of the first aspect of the present application, the tempering includes: using a double-stand tempering machine for processing, a total tempering reduction rate of 0.8%-1.5%, and a tempering speed of 200-400 m / min.

[0011] According to the embodiment of the first aspect of the present application, in the double-stand tempering machine, the reduction rate of the first stand is 0.6-0.9%, and the reduction rate of the second stand is 0.4-0.6%.

[0012] According to the embodiment of the first aspect of the present application, after the step of tempering the annealed steel plate, the operation of rewinding and oiling and the operation of dividing the coil are further included.

[0013] In the second aspect, the embodiment of the present application provides a cold-rolled ultra-low-carbon stamping soft steel, which is produced by the production method in the embodiment of the first aspect of the present application, and includes the following components in terms of mass percentage: C: ≤0.005%, Si: 0-0.05%, Mn: 0-0.3%, P: 0-0.02%, S: 0-0.018%, Alt: 0.02-0.06%, N: 0-0.005%, and the balance of Fe and unavoidable inclusions.

[0014] According to the embodiment of the second aspect of the present application, the yield strength of the cold-rolled ultra-low-carbon stamping soft steel is ≤200 MPa, the tensile strength is 270-350 MPa, and the elongation is ≥36%.

[0015] The cold-rolled ultra-low-carbon stamping soft steel plate prepared by the production method of the cold-rolled ultra-low-carbon stamping soft steel based on the bell-type annealing in the embodiment of the application has excellent forming performance, low yield strength and good plate shape. The ultra-low-carbon steel in the embodiment of the application realizes excellent forming performance and low yield strength under the condition of not adding noble metal elements Nb, V and Ti, and has lower cost through reasonable process control of the ultra-low composition of C.

[0016] Through reasonable formula design and process control, the forming performance of the steel plate is improved to meet the stamping forming requirements of the embossed security door, the yield strength of the steel plate is reduced to meet the flatness requirements of the door surface after stamping of the embossed security door, and the bell-type annealing process is adopted for production, compared with the continuous annealing process production, the bell-type annealing equipment has low investment and strong production flexibility, and does not need transition material. DETAILED DESCRIPTION

[0017] Each embodiment or implementation in the specification is described in a progressive manner, and each embodiment focuses on the difference from other embodiments.

[0018] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the application. In the specification, the exemplary description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0019] In addition, the terms "first", "second" are only used for description purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features limited by "first", "second" can explicitly or implicitly include at least one feature. In the description of the application, the meaning of "multiple" is at least two, for example, two, three, etc., unless otherwise specifically limited.

[0020] The most commonly used stamping soft steel in the market is generally low carbon steel or interstitial-free steel, such as the document with the authorized announcement number CN103469060B, which discloses a kind of cold-rolled steel sheet for household appliance stamping and its manufacturing method, the mass percentage of the chemical composition of the material of the invention is: C: 0.03~0.05%, Si≤0.03%, Mn: 0.10~0.25%, P≤0.015%, S≤0.008%, Alt: 0.022~0.038%, the rest is Fe and inclusions, which is low carbon steel. But this kind of product is mainly used for stamping parts with relatively small deformation, some large workpieces such as embossed security doors need to be made of stamping soft steel, because the plasticity of low carbon steel is relatively low, stamping embossed security doors with complex patterns will crack, in addition, because the yield strength of low carbon soft steel is relatively high, and the embossed security door mold does not have edge pressing equipment, so it is easy to cause uneven door surface (wrinkling), which affects the beauty of the embossed security door; another kind is interstitial-free steel, that is, in order to improve the stamping performance, Nb, V, Ti and other alloys are added to the steel, such as the document with the authorized announcement number CN102653839B, the production cost of the material of the invention is greatly increased because of the addition of precious alloy.

[0021] In view of the problems existing in the prior art, the present application provides a production method of cold-rolled ultra-low carbon stamping soft steel and cold-rolled ultra-low carbon stamping soft steel, which effectively solves the problems of cracking and uneven quality of low carbon steel for large workpieces such as embossed security doors, and reduces the cost without using high-priced micro-alloy elements, and the performance of the steel plate can meet the quality requirements of embossed security door stamping.

[0022] In the first aspect, the present application provides a production method of cold-rolled ultra-low carbon stamping soft steel based on cover annealing, which comprises the following steps: Continuous casting to obtain a casting blank, the casting blank comprising the following components in mass percentage: C:≤0.005%, Si:0~0.05%, Mn:0~0.3%, P:0~0.02%, S:0~0.018%, Alt:0.02~0.06%, N:0~0.005%, the balance being Fe and unavoidable inclusions; Heating the casting blank to 1180~1250℃ to carry out finish rolling in the single-phase austenite region to obtain a finish rolling steel plate, and the finish rolling temperature is 870~920℃; Coiling the finish rolling steel plate to obtain a steel coil, and the coiling temperature is 640~730℃, and the coiling speed is 1.2~2.0m / s; Cold rolling the steel coil to obtain a cold-rolled steel plate, and the cumulative reduction rate is controlled to be 60~80%; Cover annealing the cold-rolled steel plate to obtain an annealed steel plate, and the cover annealing temperature is 670~710℃, and the holding time is 5h-10h; The annealed steel plate is flattened to obtain a cold-rolled ultra-low-carbon stamping soft steel.

[0023] In view of the need for embossing forming of large workpieces such as security doors, it is required that the steel plate has excellent forming performance and does not crack in the stamping process, and that the steel plate has a relatively low yield strength and a flat door surface after stamping. Therefore, in the present application, an ultra-low-carbon composition design idea is adopted. First, the content of the interstitial solid solution element C is strictly controlled to ensure that the steel plate has excellent deep drawing forming performance. Second, the present application uses a cover annealing process for long-time heat preservation to make the solid solution element C precipitate, thereby reducing the yield strength of the steel plate while ensuring excellent aging resistance of the steel plate.

[0024] The cold-rolled ultra-low-carbon stamping soft steel plate in the embodiments of the present application has excellent forming performance, low yield strength, and good plate shape. In the embodiments of the present application, without adding valuable metals such as Nb, V, and Ti elements, through reasonable process control of the ultra-low composition of C, excellent forming performance and low yield strength are achieved, and the cost is lower.

[0025] The embodiments of the present application adopt reasonable formula design and process control to improve the forming performance of the steel plate, meet the embossing stamping forming requirements of the security door, reduce the yield strength of the steel plate, and meet the flatness requirements of the door surface of the embossed security door after stamping. The cover annealing process is used for production. Compared with the continuous annealing process production, the cover annealing equipment has low investment and strong production flexibility, and does not require transition materials.

[0026] The cold-rolled ultra-low-carbon stamping soft steel in the embodiments of the present application is not only suitable for embossing forming of security doors, but also can be used for other embossing forming purposes, and the present application is not limited thereto.

[0027] In the step of obtaining a casting blank by continuous casting, the steel plate softness and high formability are laid by controlling the chemical composition. Specifically, the key chemical composition of the steel plate in the embodiments of the present application plays the following roles: C: ≤0.0050%, C is an interstitial atom, which can significantly increase the strength of the steel plate and reduce the plasticity when existing in the form of solid solution. The main purpose of strictly controlling the content of C in the present application is to reduce the adverse effects of C on forming, thereby improving the forming performance and reducing the yield strength.

[0028] Si: 0~0.05%, Si is not only a deoxidizing element but also a solid solution strengthening element, which increases the yield strength of the product and reduces the elongation. When the silicon content is high, it is easy to generate iron oxide scale that is difficult to acid wash, which affects the surface quality of the steel plate. Therefore, the lower the Si content, the better.

[0029] Mn: 0~0.30%, manganese is a material strengthening element like silicon. With the increase of Mn content, the material strength is higher and the plasticity is lower. Therefore, the lower the Mn content, the better.

[0030] P: 0~0.02%, phosphorus can improve the strength of the material, but phosphorus can cause the material cold brittleness phenomenon, therefore, the P content as low as possible.

[0031] S: 0~0.018%, S is harmful to the steel element, S and Mn, Fe element will cause the formation of compounds in the hot rolling process of steel edge crack defects, and sulfide inclusion will also deteriorate the deep drawing performance of the steel plate, therefore, the lower the better S.

[0032] Alt: 0.02~0.06%, aluminum as a deoxidizer in steelmaking, less than 0.02% steel inclusions increase, stamping performance is poor; but too much Al will lead to the production cost, therefore, Al should be controlled in 0.02%~0.06%.

[0033] N: 0~0.005%, N and C are the same belong to interstitial atoms, when in solid solution state has significant strengthening effect, make the steel plate strength increases, plasticity decreases, should be lower and lower.

[0034] In summary, the application embodiment of the present application, the content of ultra low C, N can reduce the solid solution atom of the matrix strengthening effect, reduce the yield strength; Alt as deoxidizer and nitride forming elements, can fix free N and refine the grain; strictly limit P, S and other harmful impurities, can avoid the plasticity decline caused by the segregation at grain boundary, for the follow-up process and product performance to provide the core guarantee.

[0035] In the finishing process, the casting blank is heated to 1180~1250℃, in the single phase austenite zone finishing rolling, get the finishing plate, the finishing rolling temperature is 870~920℃. Among them, the heating temperature of 1180~1250℃ can ensure that the organization of the casting blank is fully austenitized, eliminate casting defects, make the composition and temperature uniform; in the single phase austenite zone finishing rolling can avoid the mixed crystal organization caused by two phase zone rolling, ensure the stability of the subsequent performance; the finishing temperature of 870~920℃ is higher than Ar3 line to ensure the full austenite organization, and avoid the problem of too thick iron oxide skin caused by too high temperature, at the same time, provide suitable initial conditions for subsequent cooling control.

[0036] Exemplarily, the casting blank is heated to 1180℃, 1190℃, 1200℃, 1210℃, 1220℃, 1230℃, 1240℃ and 1250℃, or any two between the range value.

[0037] Exemplarily, the finishing rolling temperature is 870℃, 880℃, 890℃, 900℃, 910℃ and 920℃, or any two between the range value.

[0038] In some embodiments, the finishing rolling process has 7 passes in total, wherein the first 3 passes have a cumulative reduction of 40-50%, the reduction of the last 4 passes is gradually reduced, and a side heating device is used after the 4th pass to make the temperature difference between the edge and the center of the cast blank ≤15℃, the water temperature at the inlet of the finishing rolling is ≤30℃, and the water temperature at the outlet is ≤45℃, and the cooling time between adjacent passes is 3-5s.

[0039] By controlling the reduction in sections, the grain size can be refined and the uniformity of the steel plate texture can be improved; the setting of the side heating device can effectively avoid the edge of the rolled piece from entering the two-phase region due to too low temperature, and reduce the risk of edge cracking during stamping.

[0040] Specifically, the large reduction of the first 3 passes can quickly break up the coarse grains of the cast blank, and the gradually reduced reduction of the last 4 passes can avoid edge cracking of the rolled piece caused by large deformation, achieving uniform control of the thickness and structure of the rolled piece; the coordination of side heating and cooling between passes can solve the problem of fast heat dissipation and low temperature of the edge of the rolled piece during finishing rolling, ensure that the entire cross section is rolled in the single-phase austenite region, avoid the formation of hard and brittle structures such as martensite in the edge, and on the other hand, reduce internal thermal stress of the rolled piece by precise temperature control, and improve the shape quality of the steel plate.

[0041] The finishing rolling needs to be completed in the single-phase austenite region, and the inlet water temperature ≤30℃ can ensure that the initial temperature of the cooling medium is relatively low, which can quickly remove the deformation heat generated between passes; the outlet water temperature ≤45℃ can avoid the cooling medium from being too hot after absorbing heat, resulting in a decrease in cooling capacity. The cooling time between adjacent passes of 3-5s can stably control the temperature of the rolled piece in the finishing rolling temperature range of 870-920℃, preventing the rolled piece from entering the overheating region due to too high temperature or falling into the two-phase region due to too low temperature, and avoiding the formation of mixed crystals, martensite and other hard and brittle structures.

[0042] In some embodiments, after the finishing rolling, the finishing rolled steel plate is also subjected to laminar cooling, and the laminar cooling adopts a cooling mode that is controlled in sections along the running direction of the finishing rolled steel plate, including a front section cooling zone and a rear section cooling zone; wherein the water spray density of the front section cooling zone is controlled to be 10-15L / (m²•s), and the water spray density of the rear section cooling zone is controlled to be 3-5L / (m²•s).

[0043] The segmented cooling mode can control the temperature of the rolled piece, avoid uneven structure caused by too fast cooling speed, and reduce the generation of surface iron oxide scale.

[0044] For example, the water spray density of the front section cooling zone is controlled to be any value or a range value between any two values in the range of 10L / (m²•s), 11L / (m²•s), 12L / (m²•s), 13L / (m²•s), 14L / (m²•s) and 15L / (m²•s).

[0045] Exemplarily, the water spraying density of the post-stage cooling zone is controlled to be any value in 3 L / (m2·s), 3.5 L / (m2·s), 4 L / (m2·s), 4.5 L / (m2·s) and 5 L / (m2·s) or a range value between any two of them.

[0046] The temperature of the steel plate after finish rolling is high, the front stage adopts a large water spraying density of 10-15 L / (m2·s) to quickly reduce the temperature of the steel plate from the finish rolling temperature to the vicinity of the phase transformation temperature interval, shortens the high-temperature residence time to reduce the surface oxidation; the post-stage switches to a small water spraying density of 3-5 L / (m2·s) to slowly reduce the temperature to the target coiling temperature, avoids the excessive temperature difference between the inside and outside of the steel plate caused by rapid cooling to generate internal stress, at the same time provides suitable temperature conditions for the dispersed precipitation of precipitates such as AlN, ensures the uniformity of the rolled structure, and improves the subsequent cold rolling and stamping performance.

[0047] In the coiling process, the coiling temperature is 640-730 ℃, and the coiling speed is 1.2-2.0 m / s.

[0048] The coiling temperature of 640-730 ℃ is in the ferrite-pearlite transformation zone, which can promote the dispersed precipitation of precipitates such as AlN, reduce the solid solution N content and refine the ferrite grains, at the same time avoid the temperature being too low to cause the hardness to rise; the coiling speed of 1.2-2.0 m / s matches the finish rolling rhythm, which can prevent the temperature of the steel plate from excessively dropping before coiling, and at the same time ensure that the coil shape is compact and the tower shape degree is small, which is convenient for subsequent cold rolling processing.

[0049] Exemplarily, the coiling temperature is any value in 640 ℃, 650 ℃, 660 ℃, 670 ℃, 680 ℃, 690 ℃, 700 ℃, 710 ℃, 720 ℃ and 730 ℃ or a range value between any two of them.

[0050] Exemplarily, the coiling speed is any value in 1.2 m / s, 1.3 m / s, 1.4 m / s, 1.5 m / s, 1.6 m / s, 1.7 m / s, 1.8 m / s, 1.9 m / s and 2.0 m / s or a range value between any two of them.

[0051] In the cold rolling process, the cumulative reduction rate is controlled to be 60-80%. The thickness reduction and texture optimization of the steel plate are realized through cold rolling, the large cumulative reduction rate of 60-80% can roll the hot-rolled steel plate to the target thickness, at the same time a large number of dislocations are generated through severe plastic deformation to provide driving force for subsequent annealing and recrystallization; in addition, the large deformation can promote the formation of beneficial stamping texture (such as {111} texture) in the ultra-low carbon steel, significantly improve the r value (plastic strain ratio) of the steel plate, enhance the anti-thinning ability in the stamping process, and reduce the risk of cracking.

[0052] However, excessive deformation will increase the rolling difficulty, and will also affect the uniformity of the structure, so that the mechanical properties are poor, so the cold rolling deformation should be controlled between 60% and 80%.

[0053] Exemplarily, the cumulative reduction of cold rolling is any value in 60%, 62%, 64%, 65%, 66%, 68%, 70%, 72%, 74%, 75%, 76%, 78% and 80% or a range value between any two of them.

[0054] In the cover annealing process, the annealing temperature is 670-710℃, and the holding time is 5h-10h. The cover annealing can eliminate work hardening and optimize the performance of the structure, and 670-710℃ is the recrystallization temperature interval, which can convert the deformation structure generated by cold rolling into fine and uniform equiaxed grains through recrystallization, completely eliminate work hardening, and reduce yield strength; The holding time of 5h-10h ensures that the temperature inside and outside the steel plate is uniform, the recrystallization is sufficient, and at the same time promotes the precipitation of carbides, reduces the adverse effects of solid solution C on stamping performance, and provides excellent plasticity basis for the steel plate.

[0055] Exemplarily, the coiling temperature is any value in 670℃, 680℃, 690℃, 700℃ and 710℃ or a range value between any two of them.

[0056] Exemplarily, the holding time is any value in 5h, 6h, 7h, 8h, 9h and 10h or a range value between any two of them.

[0057] In some embodiments, the cover annealing is full-hydrogen cover furnace annealing, and the working parameters include: the temperature rising rate from room temperature to 400℃ is 50-80℃ / h, the temperature rising rate from 400℃ to the annealing temperature is 30-50℃ / h, and the hydrogen flow rate in the holding stage is 50-80m³ / h.

[0058] The full-hydrogen atmosphere can improve the annealing uniformity and protect the surface quality of the steel plate, and the stepwise temperature rising can avoid the deformation of the steel plate caused by sudden temperature rising. The thermal conductivity of hydrogen is much higher than that of nitrogen, and the full-hydrogen atmosphere can make the steel plate heat more uniformly, avoiding the uneven grain size caused by local temperature difference.

[0059] The faster temperature rising rate from room temperature to 400℃ can improve the production efficiency, and the lower temperature rising rate after 400℃ can avoid deformation or cracking caused by heat stress concentration in the steel plate; The hydrogen flow rate of 50-80m³ / h in the holding stage can timely remove the trace amount of oxidation products generated in the annealing process, maintain a clean atmosphere in the furnace, and at the same time ensure that the hydrogen is in full contact with the surface of the steel plate, improve the annealing quality, and provide protection for the formation of excellent plasticity of the steel plate.

[0060] Exemplarily, the temperature rising rate from room temperature to 400°C is any value or a range value between any two values in the range of 50°C / h, 55°C / h, 60°C / h, 65°C / h, 70°C / h, 75°C / h and 80°C / h.

[0061] Exemplarily, the temperature rising rate from 400°C to the annealing temperature is any value or a range value between any two values in the range of 30°C / h, 35°C / h, 40°C / h, 45°C / h and 50°C / h.

[0062] Exemplarily, the hydrogen flow rate in the holding stage is any value or a range value between any two values in the range of 50 m³ / h, 55 m³ / h, 60 m³ / h, 65 m³ / h, 70 m³ / h, 75 m³ / h and 80 m³ / h.

[0063] In some embodiments, after the holding stage ends, the cold-rolled steel plate is cooled to below 180°C out of the furnace at a cooling rate of ≤28°C / h.

[0064] Slow cooling can promote the precipitation of carbon elements, reduce the adverse effects of solid-solution carbon on formability, and reduce internal stress. In the holding stage, carbon elements in the steel plate have begun to precipitate, and slow cooling at the above-mentioned cooling rate can prolong the precipitation time of carbon elements, allowing them to fully form carbides and disperse, avoiding the aggregation of solid-solution carbon atoms at the grain boundaries, which can cause a decrease in plasticity. At the same time, slow cooling can effectively reduce the temperature difference between the inside and outside of the steel plate, reduce the generation and accumulation of thermal stress, prevent the occurrence of plate shape defects such as warping and deformation due to the release of internal stress after the steel plate is taken out of the furnace, and ensure that the flatness of the steel plate meets the subsequent processing requirements.

[0065] The leveling process can improve the plate shape and eliminate the yield platform. The annealed steel plate has certain plate shape defects, and leveling can correct the plate shape through slight plastic deformation to ensure that the flatness of the steel plate meets the subsequent stamping requirements.

[0066] In some embodiments, leveling includes: using a double rack leveler for processing, and the total leveling reduction rate is 0.8% to 1.5%, and the leveling speed is 200 to 400 m / min.

[0067] Double rack leveling can achieve precise control of the reduction rate. The annealed steel plate has a significant yield platform, and direct use in stamping can easily cause Lüder lines, affecting product appearance. A total leveling reduction rate of 0.8% to 1.5% can eliminate the yield platform through slight plastic deformation, while not significantly reducing the plasticity of the steel plate. The double rack configuration is more easily able to achieve precise regulation of the reduction rate than a single rack, avoiding local work hardening caused by a large reduction rate of a single rack. A leveling speed of 200 to 400 m / min matches the pace of the production line, ensuring both the leveling effect and production efficiency, and ensuring the surface quality and plate shape precision of the steel plate.

[0068] Exemplarily, the total flattening reduction rate is any value among 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4% and 1.5% or a range value between any two of them.

[0069] Exemplarily, the flattening speed is any value among 200 m / min, 220 m / min, 240 m / min, 250 m / min, 260 m / min, 280 m / min, 300 m / min, 320 m / min, 340 m / min, 350 m / min, 360 m / min, 380 m / min and 400 m / min or a range value between any two of them.

[0070] In some embodiments, in the double-rack flattening machine, the reduction rate of the first rack is 0.6-0.9%, and the reduction rate of the second rack is 0.4-0.6%.

[0071] The segmented flattening mode can reduce the work hardening caused by the large reduction rate of the single rack and ensure the plasticity of the steel plate.

[0072] By adopting the segmented reduction strategy of large front and small back, the first rack mainly undertakes the role of eliminating the yield platform and correcting the shape of the plate with a large reduction rate of 0.6-0.9% to rapidly improve the basic performance of the steel plate; the second rack refines the steel plate with a small reduction rate of 0.4-0.6% to further optimize the shape and surface quality of the plate, while avoiding the aggravation of the internal work hardening of the steel plate caused by the large reduction rate of the single rack, ensuring that the steel plate maintains excellent plasticity and stamping performance to meet the stamping demand of complex patterns.

[0073] Exemplarily, the reduction rate of the first rack is any value among 0.6%, 0.7%, 0.8% and 0.9% or a range value between any two of them.

[0074] Exemplarily, the reduction rate of the second rack is any value among 0.4%, 0.5% and 0.6% or a range value between any two of them.

[0075] In some embodiments, after the step of flattening the annealed steel plate, the method further comprises the steps of recoiling, oiling and dividing the steel plate.

[0076] The surface of the flattened steel plate is clean but easy to oxidize and rust, and the oiling treatment during the recoiling process can form a uniform oil film on the surface of the steel plate, effectively isolating air and moisture and achieving long-term rust prevention; the oil agent (such as volatile stamping oil) can play a lubricating role during subsequent stamping, reducing the friction between the steel plate and the die and avoiding defects such as scratches and cracks during stamping; exemplarily, the dividing operation can cut the large steel coil into standard sizes of 1000-2000 mm wide and 2000-3000 mm long, which is suitable for the production equipment and processing demand of downstream products such as embossed security doors, and improves the convenience of use.

[0077] Hereinafter, the cold-rolled ultra-low carbon stamping soft steel and the production method thereof of the present application are explained in more detail through examples, but the present application is by no means limited to these examples.

[0078] Examples 1~5 A cold-rolled ultra-low carbon stamping soft steel, the production method thereof comprising the following steps: (1) smelting and continuous casting, adopting converter blowing, LF refining, RH vacuum degassing process, RH vacuum degassing vacuum degree 60 Pa, treatment time 15-20 min, obtaining a casting blank by continuous casting, the casting blank comprising the following components in mass percentage: C: ≤0.005%, Si: 0-0.05%, Mn: 0-0.3%, P: 0-0.02%, S: 0-0.018%, Alt: 0.02-0.06%, N: 0-0.005%, the balance being Fe and unavoidable inclusions; (2) heating the casting blank to 1180-1250 ℃ to carry out 7 passes of finish rolling in the single-phase austenite zone to obtain a finish-rolled steel plate, the finish rolling temperature being 870-920 ℃; wherein the cumulative reduction of the first 3 passes is 45%, the reduction of the last 4 passes gradually decreases, and the reduction of the last pass is 20%; the edge heating device is started after the fourth pass, the power is 140 kW, and the temperature difference between the edge and the center is ≤15 ℃; the water temperature at the entrance of the finish rolling is 28 ℃, the water temperature at the exit is 42 ℃, and the cooling time between adjacent passes is 4 s; (3) after finish rolling, laminar cooling is carried out in a partitioned control mode, the water spraying density in the front cooling zone is 12 L / (m²•s), and the water spraying density in the rear cooling zone is 4 L / (m²•s), the steel plate is cooled to 640-730 ℃ for coiling to obtain a steel coil, and the coiling speed is 1.4-1.6 m / s; (4) cold rolling the steel coil to obtain a cold-rolled steel plate, and the total reduction of cold rolling is 60%-80%.

[0079] (5) full-hydrogen bell annealing the cold-rolled steel plate to obtain an annealed steel plate, the temperature of the bell annealing being 670-710 ℃, and the holding time being 5 h-10 h; wherein the temperature rising rate from room temperature to 400 ℃ is 60 ℃ / h, the temperature rising rate from 400 ℃ to the target annealing temperature is 40 ℃ / h, the holding time is 5 h-10 h, the hydrogen flow rate during the holding stage is 65 m³ / h (in the range of 50-80 m³ / h), the hydrogen purity is 99.995%, and after the holding ends, the furnace is cooled to below 180 ℃ for discharging, and the cooling rate is ≤28 ℃ / h; (6) flatting the annealed steel plate by using a double-stand flatting mill to obtain a cold-rolled ultra-low carbon stamping soft steel, and the reduction of flatting is 0.8%-1.5%. Wherein, the reduction of the first stand is 0.6-0.9%, the reduction of the second stand is 0.4-0.6%, the total reduction is 0.8%-1.5%, and the flatting speed is 300 m / min; (7) Rewinding, oiling, and rewinding operations.

[0080] Comparative Examples 1-3 A cold-rolled ultra-low carbon stamping mild steel was prepared using a method similar to that of Example 1, except that some parameters were adjusted.

[0081] The components and their contents included in the billets of Examples 1-5 are detailed in Table 1, and the components and their contents included in the billets of Comparative Examples 1-3 are detailed in Table 1.

[0082] Table 1. Components and their contents in Examples 1-5 and Comparative Examples 1-3 The main process parameters for Examples 1-5 are detailed in Table 2, and the main process parameters for Comparative Examples 1-3 are detailed in Table 2.

[0083] Table 2 Main process parameters in Examples 1-5 and Comparative Examples 1-3 The performance indicators of Examples 1-5 are detailed in Table 3, and the performance indicators of Comparative Examples 1-3 are detailed in Table 3.

[0084] Table 3 Performance indicators of Examples 1-5 and Comparative Examples 1-3 The application of steel grades in Examples 1-5 is detailed in Table 4, and the application of steel grades in Comparative Examples 1-3 is detailed in Table 4.

[0085] Table 4. Application of steel grades in Examples 1-5 and Comparative Examples 1-3 In summary, the cold-rolled ultra-low carbon stamping mild steel sheet in this embodiment exhibits excellent formability, low yield strength, and good sheet shape. The ultra-low carbon steel in this embodiment achieves excellent formability and low yield strength, while also reducing cost, through ultra-low carbon content and reasonable process control, without the addition of precious metals Nb, V, and Ti.

[0086] By adopting a reasonable formula design and process control, the forming performance of steel plates is improved to meet the stamping requirements of embossed security doors, and the yield strength of steel plates is reduced to meet the flatness requirements of the door surface after stamping. The production process adopts a hood annealing process, which has lower equipment investment, greater production flexibility, and no need for transition materials compared to continuous annealing process.

[0087] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for producing a cold-rolled ultra-low carbon press-hardenable soft steel based on a bell annealing, characterized in that, The method comprises the following steps: continuous casting to obtain a casting blank, the casting blank comprising the following components in mass percentage: C: ≤0.005%, Si: 0~0.05%, Mn: 0~0.3%, P: 0~0.02%, S: 0~0.018%, Alt: 0.02~0.06%, N: 0~0.005%, the balance being Fe and inevitable inclusions; heating the casting blank to 1180~1250℃ to perform finish rolling in a single-phase austenite zone to obtain a finish-rolled steel plate, the finish rolling having a finish rolling temperature of 870~920℃; coiling the finish-rolled steel plate to obtain a steel coil, the coiling having a coiling temperature of 640~730℃ and a coiling speed of 1.2~2.0m / s; cold rolling the steel coil to obtain a cold-rolled steel plate, the cumulative reduction being controlled to be 60~80%; mask annealing the cold-rolled steel plate to obtain an annealed steel plate, the mask annealing having a temperature of 670~710℃ and a holding time of 5h-10h; flatting the annealed steel plate to obtain a cold-rolled ultra-low-carbon stamping soft steel.

2. The production method of the lid-type annealing-based cold-rolled ultra-low carbon press soft steel according to claim 1, characterized in that, The finish rolling has a total of 7 passes, wherein the first 3 passes have a cumulative reduction of 40~50%, the reduction of the last 4 passes is gradually reduced, and an edge heating device is used after the 4th pass to make the temperature difference between the edge and the center of the casting blank ≤15℃, the water temperature at the inlet of the finish rolling is ≤30℃, the water temperature at the outlet is ≤45℃, and the cooling time between adjacent passes is 3~5s.

3. The production method of lid annealing based cold rolled ultra-low carbon soft steel of claim 1, characterized in that, After the finish rolling, the finish-rolled steel plate is subjected to laminar cooling, the laminar cooling adopts a cooling mode that is controlled in zones along the running direction of the finish-rolled steel plate, and comprises a front cooling zone and a rear cooling zone; wherein the water spraying density of the front cooling zone is controlled to be 10~15L / (m²·s), and the water spraying density of the rear cooling zone is controlled to be 3~5L / (m²·s).

4. The production method of lid annealing-based cold-rolled ultra-low carbon press soft steel according to claim 1, characterized in that, The mask annealing is full-hydrogen mask furnace annealing, and the working parameters include: the temperature rising rate from room temperature to 400℃ is 50~80℃ / h, the temperature rising rate from 400℃ to the annealing temperature is 30~50℃ / h, and the hydrogen flow rate during the holding stage is 50~80m³ / h.

5. The production method of lid annealing based cold rolled ultra-low carbon soft steel of claim 4, characterized in that, After the holding ends, the cold-rolled steel plate is cooled to ≤180℃ and discharged from the furnace at a cooling rate of ≤28℃ / h.

6. The production method of lid annealing based cold rolled ultra-low carbon soft steel of claim 1, characterized in that, The flatting includes: using a double-stand flatting mill for processing, the total flatting reduction being 0.8%~1.5% and the flatting speed being 200~400m / min.

7. The production method of lid annealing based cold rolled ultra-low carbon soft steel of claim 1, characterized in that, In the double-stand flatting mill, the reduction of the first stand is 0.6~0.9%, and the reduction of the second stand is 0.4~0.6%.

8. The production method of the cold-rolled ultra-low carbon soft steel for press forming based on the bell annealing according to any one of claims 1 to 7, characterized in that, After the step of flatting the annealed steel plate, re-coiling, oiling and dividing the coil are further included.

9. A cold-rolled ultra-low carbon press-hardenable soft steel, characterized in that, The cold-rolled ultra-low-carbon stamping soft steel is produced by the production method of any one of claims 1~8, and comprises the following components in mass percentage: C: ≤0.005%, Si: 0~0.05%, Mn: 0~0.3%, P: 0~0.02%, S: 0~0.018%, Alt: 0.02~0.06%, N: 0~0.005%, the balance being Fe and inevitable inclusions.

10. Cold rolled ultra-low carbon soft steel for stamping according to claim 9, characterized in that, The cold-rolled ultra-low carbon soft steel has a yield strength of ≤ 200 MPa, a tensile strength of 270-350 MPa, and an elongation of ≥ 36%. The cold-rolled ultra-low carbon soft steel has a yield strength of ≤ 200 MPa, a tensile strength of 270-350 MPa, and an elongation of ≥ 36%.

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