High-strength low-temperature impact-resistant cold-rolled ultra-low-carbon steel and method for preparing same

Abstract

The application discloses a high-strength low-temperature impact-resistant cold-rolled ultra-low-carbon steel and a preparation method thereof, and comprises the following components in percentage by mass: C: 0.0020%-0.0040%, Si: ≤0.20%, Mn: 0.5%-0.8%, P: 0.050%-0.080%, S: ≤0.010%, Nb: 0.020%-0.050%, Ti: ≤0.008%, B: ≤0.0009%, and the balance of iron and inevitable impurities. The application designs process coupling type technical parameters of the whole process and multiple processes, and through the production process cooperation of molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing and the alloy component design, the high strength of the strength level of 380 MPa is ensured, the product integrity is still maintained when impact is carried out under the condition of lower than-80 DEG C, and the application is favorable for use in extremely cold weather in the north pole and the like.

Description

Technical Field

[0001] This invention relates to the fields of materials and metallurgy, and more specifically, to a high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel and its preparation method. Background Technology

[0002] Currently, cold-rolled ultra-low carbon steel is widely used in vehicle bodies, such as 170P1, HC220Y, 250P1, DC04 and DC06. These steel grades are now widely used in complex stamping body panels such as front hoods and dashboards.

[0003] Testing revealed that existing phosphorus-containing ultra-low carbon steel has a low-temperature impact resistance temperature of 0℃. However, below 0℃, it is prone to impact brittle fracture. This is because the grain boundary bonding force rapidly decreases after freezing below 0℃, leading to limitations in impact fracture at low temperatures. This results in numerous low-temperature impact fracture problems for customers during application, causing head-and-tail brittle fracture during the cold rolling process of ultra-low carbon steel in winter, resulting in significant cold rolling losses and affecting subsequent applications by automotive OEMs. Therefore, for the producers and users of low-temperature impact-resistant ultra-low carbon steel, improving the low-temperature impact resistance of cold-rolled ultra-low carbon steel has become a crucial research direction, providing solid technical support for next-generation automotive engineering.

[0004] In the prior art, Chinese patent application number 202010572388.8 discloses a strip steel with a tensile strength of 340MPa and a production method that has excellent low-temperature secondary processing performance. However, this product cannot meet the requirements of 380MPa strength products. At the same time, the Ti content of this invention is 0.02% to 0.06%, which will generate a large amount of FeTiP, reducing strength and increasing cold brittleness of the steel plate, and has weak resistance to low-temperature processing. The B content is 0.0003% to 0.0010%, which will generate a lot of borides. Boron has strong hardenability and reduces the elongation of cold-rolled products. In addition, the minimum R value of this invention is 1.7, which is difficult to meet the requirements of car manufacturers for an R value of ≥2.0. This is not conducive to customers' more complex stamping processing and other applications, and is prone to stamping cracks.

[0005] Chinese patent application number 202111329449.9 discloses a method for producing high-strength, low-temperature impact-resistant phosphorus-containing high-strength steel strip. However, this product does not possess the ability to maintain its integrity when subjected to impact at temperatures below -80°C. Furthermore, the Ti content of this invention is 0.02% to 0.05%, which generates a large amount of FeTiP, reducing strength, increasing cold brittleness of the steel plate, and decreasing its resistance to low-temperature impact. Simultaneously, the B content is 0.0008% to 0.0020%, which generates a large amount of borides. Boron has strong hardenability, reducing the elongation of cold-rolled products. In addition, it is not conducive to customers' complex stamping processes and is prone to stamping cracks. At the same time, the macromolecular structure of FeTiP and borides occupies the grain boundaries, forming an isolation layer without forming connecting bonds. As a result, at low temperatures below 0°C, the molecular activity is low and the grain boundary connectivity is poor, making low-temperature impact brittle fracture more likely to occur.

[0006] Chinese patent application number 201410574807.6 discloses a method for producing low-temperature impact resistant cold-based structural grade galvanized steel strip. The product subject of this invention is mainly galvanized products, and the application scenarios are different from those of cold-rolled and annealed products. Therefore, the design starting point and basic mechanism of the composition and process parameters are not the same. Moreover, this invention does not disclose the mechanical properties of the product. Based on the analysis of the published data, its mechanical properties are not in the same strength level as those of this invention. At the same time, the product has a low carbon design (C: 0.02%-0.06%), poor processing and ductility, and cannot meet the requirements of complex deformation and deep processing. It also does not meet the low-temperature impact resistance of -80℃.

[0007] Therefore, for ultra-low carbon steel, the effect of using a single control method is not obvious. It is necessary to suppress it based on the coupling design of the whole process. Therefore, it is of great significance to explore a preparation method for low-temperature impact resistance based on the whole process of composition design, steelmaking, hot rolling, cold rolling, pickling and annealing. Summary of the Invention

[0008] The purpose of this invention is to overcome the aforementioned defects in the existing technology and provide a high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel and its preparation method. This invention designs a multi-stage, process-coupled technical parameter system. Through a production process of "molten iron pretreatment + converter smelting + RH refining + continuous casting + hot rolling + cold rolling + annealing" combined with alloy composition design, the resulting ultra-low carbon steel cold-rolled products maintain their integrity when subjected to impacts at temperatures ranging from 0°C to below -80°C, without exhibiting impact brittle fracture. This avoids the impact brittle fracture that has occurred with various types and compositions of ultra-low carbon steel cold-rolled products in the past, thus providing the ability to resist impact fracture in low-temperature applications. Furthermore, while increasing processing deformation capacity, the addition of phosphorus, manganese, niobium, and other alloys forms bonding bonds at grain boundaries, strengthening grain boundary connections. Simultaneously, the two-phase NbCN particles refine the grains, increasing product strength and enhancing the grain boundary's resistance to low-temperature impact. In addition, an R-value ≥ 2.0 is beneficial for customers to process parts using deep-drawing methods with complex deformation.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] A high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel comprises the following components by mass percentage: C: 0.0020%–0.0040%, Si: ≤0.20%, Mn: 0.5%–0.8%, P: 0.050%–0.080%, S≤0.010%, Nb: 0.020%–0.050%, Ti≤0.008%, B≤0.0009%, and the balance being iron and unavoidable impurities.

[0011] The present invention also discloses a method for preparing high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel as described above, comprising the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling, and annealing;

[0012] In the aforementioned molten iron pretreatment, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron;

[0013] In the converter smelting process, the desulfurized molten iron is smelted in a converter and then tapped into steel.

[0014] In the RH refining process, the molten steel after being tapped from the converter smelting furnace is subjected to RH refining treatment.

[0015] In the continuous casting process, the molten steel that has undergone RH refining is continuously cast to obtain a continuously cast billet.

[0016] In the hot rolling process, the heating temperature is 1200℃~1260℃, the roughing mill exit temperature and the finishing mill start temperature are both 1040℃~1080℃, the hot rolling finish temperature is 910℃~970℃, the laminar flow cooling is set as a front-end centralized cooling form, the coiling temperature is 680℃~760℃, and after the process coiling, the hot coils are cooled by stacking them, and the hot coil cooling rate is controlled at 10℃ / h~20℃ / h to obtain hot-rolled steel coils.

[0017] In the cold rolling process, the hot-rolled steel coil is pickled and then cold-rolled to a target thickness of 0.5 mm to 2.0 mm.

[0018] In the annealing process, the cold-rolled steel is continuously annealed to obtain the cold-rolled ultra-low carbon steel.

[0019] Implementing the embodiments of the present invention will have the following beneficial effects:

[0020] (1) The production process control scheme for cold-rolled ultra-low carbon steel with a low temperature impact resistance of 380MPa provided by the present invention breaks the previous status quo that ultra-low carbon steel has poor impact resistance at low temperature ≤0℃. It proposes a multi-process coupled design to improve the low temperature impact resistance of ultra-low carbon steel and achieves no fracture at low temperature impact of -80℃.

[0021] (2) The production process of the present invention does not require additional manufacturing equipment, has the characteristics of low cost and high efficiency, and is easy to industrialize and mass-produce. It has good application effect and can be industrialized without other auxiliary equipment and special process equipment, which simplifies the production process and reduces production costs.

[0022] (3) Through composition design and combined with steelmaking, hot rolling, pickling, and annealing processes, this invention uses Nb in the form of microalloyed two-phase particles to enhance the product's resistance to low-temperature impact. This allows the cold-rolled ultra-low carbon steel of this invention to maintain its integrity when subjected to impacts at temperatures ranging from 0°C to below -80°C, without exhibiting cold brittle fracture. At the same time, controlling the Ti content to ≤0.008% and the B content to ≤0.0009% avoids the formation of large amounts of FeTiP precipitates and borides in Ti and B, which weaken grain boundary bonding, and prevents the formation of fracture layers in Ti and B at ultra-low temperatures. Furthermore, through analysis of the cold-rolled ultra-low carbon steel product design of this invention, while increasing processing deformation capacity, phosphorus, manganese, niobium, and other alloys are added. These elements form reinforcing bonds at grain boundaries, improving strength. Meanwhile, a large amount of fine NbCN two-phase particles refine the grains, increasing the product's strength to over 380MPa, meeting customers' demand for high-strength products above 380MPa, and improving the impact resistance of customer products in low-temperature environments.

[0023] (4) The production method of the ultra-low carbon cold-rolled product with low-temperature impact resistance of the present invention differs from the previous process control in product development. It designs the process parameters for multiple steps throughout the entire process to achieve control over the low-temperature impact resistance of cold-rolled ultra-low carbon steel. The cold-rolled ultra-low carbon steel produced using the production method provided by the present invention has a tensile strength of over 380 MPa and a yield strength of 260 MPa to 320 MPa; the elongation of the cold-rolled ultra-low carbon steel is A. 50 The strength is 38%–48%; R-value ≥ 2.0. It also does not fracture under impact at -60℃ or -80℃.

[0024] The cold-rolled ultra-low carbon steel not only ensures a high strength of 380MPa, but also ensures that the product maintains its integrity when subjected to impact at temperatures below -80℃, which is beneficial for use in extreme cold weather such as the Arctic. At the same time, the Ti content ≤0.008% and the B content ≤0.0009% are residual contents, which avoids the formation of FeTiP and borides due to the addition of Ti and B, thus avoiding the impact on strength and elongation, and increasing the ability to resist low-temperature impact. Attached Figure Description

[0025] Figure 1 This is a distribution diagram of NbCN two-phase particles in steel according to Example 1 of the present invention. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0027] I. Chemical composition and mechanical properties

[0028] This invention discloses a high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel, comprising the following components by mass percentage: C: 0.0020%–0.0040%, Si: ≤0.20%, Mn: 0.5%–0.8%, P: 0.050%–0.080%, S≤0.010%, Nb: 0.020%–0.050%, Ti≤0.008%, B≤0.0009%, and the balance being iron and unavoidable impurities.

[0029] Specifically, the mechanism of action of each alloy component in the cold-rolled ultra-low carbon steel of the present invention is as follows:

[0030] Carbon content (C): 0.0020%–0.0040%. Ultra-low carbon design increases product ductility, enabling complex deformation applications, while also enhancing resistance to low-temperature impacts. If the C content exceeds this range, an adequate amount of two-phase particles (NbCN) cannot be generated, affecting both the product's low-temperature impact resistance and its processing performance (R).

[0031] A Si content of ≤0.20% is beneficial for increasing the product's ductility, improving its R-value, and enhancing its resistance to low-temperature shock. Excessive Si content beyond this range leads to lattice distortion, hindering the product's ability to function as a lattice barrier and negatively impacting its processing and ductility.

[0032] Mn content of 0.5%–0.8% is designed to increase the tensile strength of the product and enhance its resistance to low-temperature impact. If the Mn content exceeds this range, the alloy cannot effectively strengthen the ferrite grains, thus affecting the product's machinability.

[0033] P (phosphorus): 0.050%–0.080%, can significantly increase tensile strength to over 380 MPa. If the P content exceeds this range, the alloy cannot effectively strengthen the ferrite grains, affecting the product's machinability.

[0034] Nb content of 0.020%–0.050% promotes the formation of a large number of fine NbCN two-phase particles in the steel, which refine the grains, increase grain boundary connectivity, enhance resistance to low-temperature impact, and simultaneously increase yield strength and tensile strength. If the Nb content exceeds this range, an adequate amount of NbCN two-phase particles cannot be formed, affecting the product's resistance to low-temperature impact and its processing performance.

[0035] Furthermore, Ti content ≤ 0.008% and B content ≤ 0.0009% are selected as residual elements to avoid the significant decrease in strength and r-value of high-strength IF steel due to the formation of a large amount of FeTiP during annealing caused by the addition of Ti, and to avoid the impact of the addition of B on yield strength and elongation. At the same time, controlling the Ti content ≤ 0.008% and the B content ≤ 0.0009% is also to avoid the formation of FeTiP and boride macromolecular structures occupying grain boundaries, forming an isolation layer without forming connecting bonds, reducing the material's resistance to low-temperature impact, and thus improving the R-value of the product.

[0036] In one specific embodiment, the cold-rolled ultra-low carbon steel has a tensile strength ≥380MPa, a yield strength of 260MPa~320MPa, and an elongation A. 50 The percentage is 38%–48%; R value ≥ 2.0.

[0037] In one specific embodiment, the cold-rolled ultra-low carbon steel does not fracture upon impact at -60°C or -80°C.

[0038] II. Production Process Technology

[0039] This invention also discloses a method for preparing high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel as described in any embodiment of this invention, comprising the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling, and annealing, as detailed below:

[0040] (1) In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron. Its function is to provide favorable conditions for the smelting of various auxiliary alloys in the later steelmaking process, thereby controlling the required S content of the finished product.

[0041] (2) In converter smelting, the desulfurized molten iron is smelted in a converter and then tapped into steel.

[0042] In one specific embodiment, step (2) specifically includes: smelting the desulfurized molten iron in a converter. The purpose is to raise the temperature and reduce the C, P, and S content in the original molten iron, providing favorable conditions for refining RH to achieve the required smelting composition of the finished product.

[0043] (3) In RH refining, the molten steel after being tapped from the converter is subjected to RH refining treatment.

[0044] In one specific embodiment, step (3) specifically includes: subjecting the molten steel after tapping from the converter to RH refining treatment, with an RH inlet temperature of 1570℃~1620℃, a temperature rise of ≤25℃, a target static time of 35min, and an outlet temperature of 1580℃~1590℃. The purpose is to treat the molten steel to achieve the required smelting composition and to achieve a suitable temperature for casting, thus providing favorable conditions for ensuring the quality of the cast billet meets customer requirements. Exceeding the temperature range can easily lead to defects such as increased inclusions.

[0045] (4) In continuous casting, molten steel that has undergone RH refining is continuously cast to obtain continuously cast billets. Its function is to form molten steel and produce billets that meet the quality requirements of customers.

[0046] In one specific embodiment, step (4) specifically includes: controlling the baking temperature of the tundish to be >1150℃, the superheat of the molten steel in the tundish to be 25℃~45℃; using a long nozzle from the ladle to the tundish and argon sealing protection, the long nozzle penetration depth is 150mm~350mm; covering the tundish with a covering agent to a depth of 210mm, the weight of the molten steel in the tundish is ≥18 tons; using an immersion nozzle from the tundish to the crystallizer and argon sealing protection, the crystallizer penetration depth of the nozzle is 120mm~220mm, and hot delivery of the continuous casting billet.

[0047] (5) In hot rolling, the heating temperature is 1200℃~1260℃, the roughing exit temperature and the finishing rolling start temperature are both 1040℃~1080℃, the hot rolling finish temperature is 910℃~970℃, the laminar flow cooling is set as the front-end centralized cooling form, the coiling temperature is 680℃~760℃, and the hot coils are cooled by stacking them after the process coiling. The hot coil cooling rate is controlled at 10℃ / h~20℃ / h to obtain hot-rolled steel coils.

[0048] Specifically, during the hot rolling process, specific processes are used to promote the precipitation of two-phase particles of appropriate specifications and types, while simultaneously promoting the formation of a texture conducive to low-temperature impact resistance. Specifically, the heating temperature is 1200℃~1260℃ to ensure sufficient heating of the billet and uniform internal and external temperatures; the roughing mill exit temperature and finishing mill initiation temperature are 1040℃~1080℃ to facilitate stable hot rolling; the hot rolling final temperature is 910℃~970℃ to ensure the product is rolled in the austenitic region, and laminar cooling is set to a front-stage centralized cooling mode to increase the grain refinement of the steel plate and improve its resistance to low-temperature impact; the coiling temperature is 680℃~760℃ to form a texture beneficial for stamping processing, improving the product's ability to handle complex processes, and simultaneously enhancing its resistance to low-temperature impact; after coiling, the hot coils are cooled in a stacked manner, with the cooling rate controlled at 10℃ / h~20℃ / h to form a grain structure and texture beneficial for stamping processing, thereby improving the product's ability to handle complex processes and its resistance to low-temperature impact.

[0049] (6) In cold rolling, hot-rolled steel coils are pickled and then cold-rolled to a target thickness of 0.5mm to 2.0mm.

[0050] In one specific embodiment, step (6) specifically includes: in cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5% to 0.8%, and after straightening, it is subjected to shallow groove turbulent pickling at a temperature of 70°C to 90°C and a pickling speed of 180 m / min to 230 m / min; the pickled steel plate is rolled into cold-rolled hard steel strip with a deformation compression rate ≥60%.

[0051] Specifically, the cold rolling pickling stage uses a stretching straightening process with an elongation of 0.5% to 0.8% followed by shallow-groove turbulent pickling to remove iron oxide scale, resulting in a smooth surface on the cold-rolled steel sheet, which is beneficial for further coating and other applications. The rolling mill rolls the hot coil into cold-rolled hard steel strip, which breaks down the grains and achieves a deformation compression rate of ≥60%. This increases the degree of grain fragmentation in the cold-rolled product, which is beneficial for recrystallization after annealing to obtain finer grains. It also helps to increase grain boundary bonding and improve resistance to low-temperature impact.

[0052] (7) During annealing, the cold-rolled steel is continuously annealed to obtain cold-rolled ultra-low carbon steel.

[0053] In one specific embodiment, step (7) specifically includes: the annealing temperature in the heating section is 760℃~850℃, the annealing temperature in the soaking section is 760℃~850℃, the slow cooling temperature is 640℃~690℃, the rapid cooling temperature is 450℃~500℃, the aging section temperature is 440℃~490℃, and the final cooling temperature is 150℃~180℃.

[0054] Specifically, continuous annealing is adopted, with a soaking temperature set at 760℃~850℃. The purpose is to re-dissolve precipitated and grown two-phase particles such as NbCN in the steel, while simultaneously promoting the development of recrystallization texture in the rolled grains. This slightly increases the steel's elongation and R90 while ensuring low-temperature impact resistance. The slow cooling temperature is set at 640℃~690℃ to facilitate the precipitation of NbCN into fine two-phase particles, hindering the development of recrystallization texture. This also ensures the steel's yield strength, tensile strength, elongation, and low-temperature impact resistance. Impact resistance; the rapid cooling temperature is set at 450℃~500℃ to further refine the grains and stabilize the precipitated phases and grain structure; the aging temperature is set at 440℃~490℃ to ensure the precipitation of fine NbCN two-phase particles in the grains while completing the annealed recrystallization transformation of the cold-rolled product, increasing R90 to ensure good stamping performance, while not inducing the precipitation of cold-brittle FeTiP, thus enhancing the ability to resist low-temperature impact; the final cooling temperature is set at 150℃~180℃ to stabilize the grain structure and prevent further changes in the steel quality.

[0055] The following are specific embodiments.

[0056] Example 1

[0057] The high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel of this embodiment comprises the following components by mass percentage: C: 0.0028%, Si: 0.124%, Mn: 0.65%, P: 0.062%, S: 0.0042%, Nb: 0.0249%, Ti: 0.0008%, B: 0.0001%, and the balance being iron and unavoidable impurities.

[0058] The preparation method of high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel in this embodiment includes the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling, and annealing, as detailed below:

[0059] (1) In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0060] (2) In converter smelting, the desulfurized molten iron is smelted in a converter.

[0061] (3) In RH refining, the molten steel after being tapped from the converter is subjected to RH refining treatment. The RH inlet temperature is 1580℃, the temperature rise is ≤25℃, the static time target is 35min, and the outlet temperature is 1583℃.

[0062] (4) In continuous casting, the baking temperature of the tundish is controlled at 1150℃ and the superheat of the molten steel in the tundish is 30℃; the long nozzle is used from the ladle to the tundish and is protected by argon, and the immersion depth of the long nozzle is 220mm; the tundish is covered with a covering agent and the weight of the molten steel in the tundish is 26 tons; the immersion nozzle is used from the tundish to the crystallizer and is protected by argon, and the immersion depth of the crystallizer nozzle is 180mm, and the continuously cast billet is hot-sent.

[0063] (5) In hot rolling, the heating temperature is 1235℃, the roughing exit temperature is 1079℃, the finishing rolling start temperature is 1068℃, the hot rolling finish temperature is 923℃, the laminar flow cooling is set as the front-end centralized cooling form, the coiling temperature is 701℃, and the hot coil is cooled by stacking hot coils after the process coiling. The hot coil cooling rate is controlled at 17℃ / h to obtain hot-rolled steel coils.

[0064] (6) In cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.7%. After straightening, it is pickled in a shallow groove with a pickling temperature of 85°C and a pickling speed of 200 m / min. The pickled steel plate is rolled into cold-rolled hard steel strip with a deformation compression rate ≥60%.

[0065] (7) During annealing, the cold-rolled steel is continuously annealed. The annealing temperature in the heating section is 789°C, the annealing temperature in the soaking section is 803°C, the slow cooling temperature is 651°C, the fast cooling temperature is 455°C, the aging section temperature is 443°C, and the final cooling temperature is 153°C.

[0066] Example 2

[0067] The high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment comprises the following components by mass percentage: C: 0.0027%, Si: 0.1351%, Mn: 0.6755%, P: 0.06407%, S: 0.00539%, Nb: 0.0262%, Ti: 0.0008%, B: 0.0001%, and the balance being iron and unavoidable impurities.

[0068] The preparation method of high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel in this embodiment includes the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling, and annealing, as detailed below:

[0069] (1) In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0070] (2) In converter smelting, the desulfurized molten iron is smelted in a converter.

[0071] (3) In RH refining, the molten steel after being tapped from the converter is subjected to RH refining treatment. The RH inlet temperature is 1602℃, the temperature rise is ≤25℃, the static time target is 35min, and the outlet temperature is 1581℃.

[0072] (4) In continuous casting, the baking temperature of the tundish is controlled at 1201℃ and the superheat of the molten steel in the tundish is 30℃; the long nozzle is used from the ladle to the tundish and is protected by argon, and the penetration depth of the long nozzle is 201mm; the tundish is covered with a covering agent and the weight of the molten steel in the tundish is 25 tons; the immersion nozzle is used from the tundish to the crystallizer and is protected by argon, and the penetration depth of the crystallizer nozzle is 149mm, and the continuously cast billet is hot-sent.

[0073] (5) In hot rolling, the heating temperature is 1243℃, the roughing exit temperature is 1077℃, the finishing rolling start temperature is 1073℃, the hot rolling finish temperature is 923℃, the laminar flow cooling is set as the front-end centralized cooling form, the coiling temperature is 702℃, and the hot coil is cooled by stacking hot coils after the process coiling. The hot coil cooling rate is controlled at 12℃ / h to obtain hot rolled steel coils.

[0074] (6) In cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.6%. After straightening, it is pickled in a shallow groove with a pickling temperature of 80°C and a pickling speed of 227 m / min. The pickled steel plate is rolled into cold-rolled hard steel strip with a deformation compression rate of ≥60%.

[0075] (7) During annealing, the cold-rolled steel is continuously annealed. The annealing temperature in the heating section is 788℃, the annealing temperature in the soaking section is 800℃, the slow cooling temperature is 646℃, the fast cooling temperature is 450℃, the aging section temperature is 448℃, and the final cooling temperature is 155℃.

[0076] Example 3

[0077] The high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment comprises the following components by mass percentage: C: 0.0029%, Si: 0.1369%, Mn: 0.6552%, P: 0.06423%, S: 0.0037%, Nb: 0.0295%, Ti: 0.0006%, B: 0.0001%, and the balance being iron and unavoidable impurities.

[0078] The preparation method of high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel in this embodiment includes the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling, and annealing, as detailed below:

[0079] (1) In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0080] (2) In converter smelting, the desulfurized molten iron is smelted in a converter.

[0081] (3) In RH refining, the molten steel after being tapped from the converter is subjected to RH refining treatment. The RH inlet temperature is 1572℃, the temperature rise is ≤25℃, the static time target is 35min, and the outlet temperature is 1589℃.

[0082] (4) In continuous casting, the baking temperature of the tundish is controlled at 1189℃ and the superheat of the molten steel in the tundish is 33℃; the long nozzle is used from the ladle to the tundish and is protected by argon, and the penetration depth of the long nozzle is 230mm; the tundish is covered with a covering agent and the weight of the molten steel in the tundish is 27 tons; the immersion nozzle is used from the tundish to the crystallizer and is protected by argon, and the penetration depth of the crystallizer nozzle is 177mm, and the continuously cast billet is hot-sent.

[0083] (5) In hot rolling, the heating temperature is 1245℃, the roughing exit temperature is 1068℃, the finishing rolling start temperature is 1072℃, the hot rolling finish temperature is 918℃, the laminar flow cooling is set as the front-end centralized cooling form, the coiling temperature is 698℃, and the hot coil is cooled by stacking hot coils after the process coiling. The hot coil cooling rate is controlled at 18℃ / h to obtain hot rolled steel coils.

[0084] (6) In cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.55%. After straightening, it is pickled in a shallow groove with a pickling temperature of 78°C and a pickling speed of 189 m / min. The pickled steel plate is rolled into cold-rolled hard steel strip with a deformation compression rate of ≥60%.

[0085] (7) During annealing, the cold-rolled steel is continuously annealed. The annealing temperature in the heating section is 791°C, the annealing temperature in the soaking section is 805°C, the slow cooling temperature is 652°C, the fast cooling temperature is 453°C, the aging section temperature is 440°C, and the final cooling temperature is 150°C.

[0086] Example 4

[0087] The high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel of this embodiment comprises the following components by mass percentage: C: 0.0032%, Si: 0.131%, Mn: 0.6816%, P: 0.063%, S: 0.0046%, Nb: 0.02655%, Ti: 0.0008%, B: 0.0001%, and the balance being iron and unavoidable impurities.

[0088] The preparation method of high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel in this embodiment includes the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling, and annealing, as detailed below:

[0089] (1) In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0090] (2) In converter smelting, the desulfurized molten iron is smelted in a converter.

[0091] (3) In RH refining, the molten steel after being tapped from the converter is subjected to RH refining treatment. The RH inlet temperature is 1602℃, the temperature rise is ≤25℃, the static time target is 35min, and the outlet temperature is 1588℃.

[0092] (4) In continuous casting, the baking temperature of the tundish is controlled at 1209℃ and the superheat of the molten steel in the tundish is 27℃; the long nozzle is used from the ladle to the tundish and is protected by argon, and the penetration depth of the long nozzle is 193mm; the tundish is covered with a covering agent and the weight of the molten steel in the tundish is 32 tons; the immersion nozzle is used from the tundish to the crystallizer and is protected by argon, and the penetration depth of the crystallizer nozzle is 172mm, and the continuously cast billet is hot-sent.

[0093] (5) In hot rolling, the heating temperature is 1256℃, the roughing exit temperature is 1075℃, the finishing rolling start temperature is 1080℃, the hot rolling finish temperature is 918℃, the laminar flow cooling is set as a front-end centralized cooling form, the coiling temperature is 709℃, and the hot coil is cooled by stacking hot coils after the process coiling. The hot coil cooling rate is controlled at 11℃ / h to obtain hot-rolled steel coils.

[0094] (6) In cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.63%. After straightening, it is pickled in a shallow groove with a pickling temperature of 87°C and a pickling speed of 213 m / min. The pickled steel plate is rolled into cold-rolled hard steel strip with a deformation compression rate ≥60%.

[0095] (7) During annealing, the cold-rolled steel is continuously annealed. The annealing temperature in the heating section is 795°C, the annealing temperature in the soaking section is 809°C, the slow cooling temperature is 655°C, the fast cooling temperature is 458°C, the aging section temperature is 455°C, and the final cooling temperature is 156°C.

[0096] Test case

[0097] The cold-rolled ultra-low carbon steels prepared in Examples 1-4 were subjected to low-temperature impact resistance and tensile impact tests, and the results are shown in Table 1.

[0098] Table 1. Results of low-temperature impact and tensile impact tests in Examples 1-4

[0099]

[0100] As can be seen from the above embodiments, the production method of cold-rolled ultra-low carbon steel with a low-temperature impact resistance of 380MPa according to the present invention can significantly improve the quality of cold-rolled ultra-low carbon steel products in terms of low-temperature impact resistance.

[0101] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel, characterized in that, Includes the following components by mass percentage: C: 0.0020%~0.0040%, Si: ≤0.20%, Mn: 0.5%~0.8%, P: 0.062%~0.080%, S≤0.010%, Nb: 0.020%~0.0295%, Ti≤0.008%, B≤0.0009%, and the balance being iron and unavoidable impurities; The tensile strength of the cold-rolled ultra-low carbon steel is ≥380MPa; The yield strength of the cold-rolled ultra-low carbon steel is 285MPa~320MPa; The elongation of the cold-rolled ultra-low carbon steel is 38%~48% for A50. R value ≥ 2.0; It does not break under impact at -60℃, and it does not break under impact at -80℃. The preparation method of the high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel includes the following steps: hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing; In the aforementioned molten iron pretreatment, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron; In the converter smelting process, the desulfurized molten iron is smelted in a converter and then tapped into steel. In the RH refining process, the molten steel after being tapped from the converter smelting furnace is subjected to RH refining treatment. In the continuous casting process, the molten steel that has undergone RH refining is continuously cast to obtain a continuously cast billet. In the hot rolling process, the heating temperature is 1200℃~1260℃, the roughing mill exit temperature and the finishing mill start temperature are both 1040℃~1080℃, the hot rolling finish temperature is 910℃~970℃, the laminar flow cooling is set as a front-end centralized cooling form, the coiling temperature is 680℃~760℃, and after the process coiling, the hot coils are cooled by stacking them, and the hot coil cooling rate is controlled at 10℃ / h~20℃ / h to obtain hot-rolled steel coils. In the cold rolling process, the hot-rolled steel coil is pickled and then cold-rolled to a target thickness of 0.5mm to 2.0mm; In the annealing process, the cold-rolled steel is continuously annealed to obtain the cold-rolled ultra-low carbon steel. In the cold rolling process, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5% to 0.8%. After straightening, it undergoes shallow-groove turbulent pickling at a temperature of 70°C to 90°C and a pickling speed of 180 m / min to 230 m / min. The pickled steel sheet is then rolled into cold-rolled hard steel strip with a deformation compression ratio ≥60%. During the annealing process, the annealing temperature in the heating section is 760℃~850℃, the annealing temperature in the soaking section is 760℃~850℃, the slow cooling temperature is 640℃~690℃, the rapid cooling temperature is 450℃~500℃, the aging section temperature is 440℃~490℃, and the final cooling temperature is 150℃~180℃.

2. The high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel according to claim 1, characterized in that, In the RH refining process, the molten steel after being tapped from the converter is subjected to RH refining treatment. The RH inlet temperature is 1570℃~1620℃, the temperature rise is ≤25℃, the static time is 35min, and the outlet temperature is 1580℃~1590℃.

3. The high-strength, low-temperature impact-resistant, cold-rolled ultra-low carbon steel according to claim 1, characterized in that, In the continuous casting process, the tundish baking temperature is controlled to be >1150℃, and the superheat of the molten steel in the tundish is 25℃~45℃; a long nozzle is used from the ladle to the tundish and is protected by argon, with an immersion depth of 150mm~350mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is ≥18 tons; an immersion nozzle is used from the tundish to the crystallizer and is protected by argon, with an immersion depth of 120mm~220mm in the crystallizer, and the continuously cast billet is hot-delivered.

Images