Low-temperature-impact-resistant 350MPa-grade ultra-low-carbon cold-rolled steel strip and preparation method thereof

Abstract

The invention discloses a low-temperature-impact-resistant 350MPa-grade ultra-low-carbon cold-rolled steel strip and a preparation method thereof. The low-temperature-impact-resistant 350MPa-grade ultra-low-carbon cold-rolled steel strip comprises the following components in percentage by mass: less than or equal to 0.0020% of C, less than or equal to 0.20% of Si, 0.20-0.49% of Mn, 0.020-0.049% of P, less than or equal to 0.008% of S, 0.010-0.030% of Nb, less than or equal to 0.008% of Ti, less than or equal to 0.0008% of B and the balance of iron and inevitable impurities. According to the invention, a whole-process multi-procedure process coupling type technical parameter design is adopted, and a production process of molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing is matched with an alloy component design, so that high strength of 350MPa is ensured, and low-temperature impact resistance below-80 DEG C can be ensured; the integrity of the product is still kept during impact, and the use in extreme cold weather such as the north pole is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more specifically, to a low-temperature impact-resistant 350MPa-grade ultra-low carbon cold-rolled steel strip and a preparation method thereof. Background Art

[0002] At present, cold-rolled ultra-low carbon steels such as 170P1, 210P1, 250P1, DC04 and DC06 have been widely used in body covering parts with more complex stamping, such as deep stamping parts such as front cover plates and automobile instrument panels.

[0003] Nowadays, the demand for building automobile processing lines in cold northern regions is increasing. For example, the winter temperature in Xinjiang, my country is -30℃~40℃, the winter temperature in Russia is -30℃~-60℃, and the winter temperature in the Arctic is -50℃~60℃. In this low-temperature environment, the use of vehicles requires resistance to low-temperature impact without fracture, but the cold rolling production process of ultra-low carbon steel in winter is prone to head and tail rolling brittle fracture, causing a large amount of cold rolling cutting and affecting the subsequent application of automobile OEMs. In addition, the low-temperature impact temperature of existing phosphorus-containing ultra-low carbon steel is only 0℃, and it is easy to have impact brittle fracture below 0℃. This is because the bonding force of the grain boundary decreases rapidly after being frozen at a low temperature below 0℃, which leads to the limitation of impact fracture at a low temperature below 0℃, causing many low-temperature impact fracture problems for customers during the application process.

[0004] In the prior art, the Chinese patent with application number 201210088829.2 discloses a method for producing ultra-low carbon high-strength cold-rolled plates, but the products obtained by this technical solution are not suitable for low-temperature environments. The addition of Ti elements will generate a large amount of FeTiP, which will reduce the strength and increase the cold brittleness of the steel plate, and the ability to resist low-temperature processing is weak; and the R value of the product of this patent is 1.74, which is not conducive to customers' more complex stamping processing and other applications, and it is easy to cause stamping cracks. The Chinese patent with application number 201911356888.1 discloses a super-tough steel material and its manufacturing method. The carbon content of this patent is too high, and the stamping performance is not good. It does not belong to the application scope of ultra-low carbon automotive deep stamping, and cannot be used on cars in severe cold and low-temperature areas; at the same time, the temperature of low-temperature impact resistance is not specified, and the degree of low-temperature impact resistance cannot be measured. The Chinese patent with application number 201910473440.1 discloses an instrument housing part resistant to low-temperature impact and its preparation method. The product of this patent is also not suitable for low-temperature environment, and the composition of the product contains 0.05-0.09% Ti and 0.0005-0.0020% B, which will generate a large amount of FeTiP to reduce the strength and increase the cold brittleness of the steel plate, and the ability to resist low-temperature processing is weak; at the same time, the B content within this range will generate more borides, and the strong hardenability of boron reduces the elongation and R value of the cold plate product, which is not conducive to customers' more complex stamping processing and other applications, and is prone to stamping cracks.

[0005] Therefore, how to improve the ability of ultra-low carbon cold-rolled steel strip to resist low-temperature impact has become a crucial research direction, providing solid technical support for the new generation of automotive engineering. Summary of the invention

[0006] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and to provide a 350MPa ultra-low carbon cold-rolled steel strip resistant to low-temperature impact and a preparation method thereof. The present invention designs the process coupling technical parameters of the whole process and multiple processes, and through the production process of "molten iron pretreatment + converter smelting + RH refining + continuous casting + hot rolling + cold rolling + annealing" combined with the alloy composition design, it not only ensures the high strength level of 350MPa, but also ensures the ability to resist low-temperature impact below -80°C.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] The invention discloses a low-temperature impact-resistant 350MPa-grade ultra-low carbon cold-rolled steel strip, which comprises the following components in mass percentage: C≤0.0020%, Si:≤0.20%, Mn: 0.20%-0.49%, P: 0.020%-0.049%, S≤0.008%, Nb: 0.010%-0.030%, Ti≤0.008%, B≤0.0008%, and the balance is iron and inevitable impurities.

[0009] The present invention also discloses a method for preparing the above-mentioned low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip, comprising the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing;

[0010] In the converter smelting, the desulfurized molten iron is subjected to converter smelting and steel is tapped;

[0011] In the RH refining, the molten steel discharged from the converter is subjected to RH refining treatment;

[0012] In the continuous casting, the molten steel subjected to the RH refining treatment is continuously cast to obtain a continuously cast ingot;

[0013] In the hot rolling, the heating temperature is 1210°C to 1270°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1050°C to 1110°C, the hot rolling final rolling temperature is 900°C to 960°C, the laminar cooling is set as a front-stage centralized cooling form, the coiling temperature is 660°C to 740°C, and the hot coil is cooled in the form of stacking after the process coiling, and the hot coil cooling rate is controlled at 7°C / h to 18°C / h to obtain a hot rolled steel coil;

[0014] In the cold rolling, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 0.3 mm to 3.0 mm;

[0015] During the annealing, the cold-rolled steel material is continuously annealed to obtain the ultra-low carbon cold-rolled steel strip.

[0016] Implementing the embodiments of the present invention will have the following beneficial effects:

[0017] (1) The present invention designs the process-coupled technical parameters of the whole process and multiple steps. Through the production process of "molten iron pretreatment + converter smelting + RH refining + continuous casting + hot rolling + cold rolling + annealing" and the alloy composition design, it not only ensures the high strength level of 350MPa, but also ensures the ability to resist low-temperature impact below -80°C.

[0018] (2) The ultra-low carbon cold-rolled steel strip produced by the production method provided by the present invention has a tensile strength of 350-420 MPa, a yield strength of 220-270 MPa, and an elongation of A 50 The product has an R value of 41-52% and an R value of ≥2.1, which can meet the customer's application requirements for complex deep stamping deformation. At the same time, it does not break when impacted at -80°C, that is, it still maintains the integrity of the product when impacted at temperatures below -80°C, and there is no cold brittle fracture. It has the ability to resist impact fracture in low-temperature applications, which is beneficial for use in cold northern regions, especially in extremely cold weather areas such as Xinjiang, China, Siberia, and the Arctic.

[0019] (3) The present invention designs the composition of the ultra-low carbon cold-rolled steel strip to design a Ti content of ≤0.008% and a B content of ≤0.0008%, thereby preventing Ti from generating a large amount of FeTiP during annealing to reduce the strength and R value of the high-strength IF steel, and preventing the addition of B from affecting the yield strength and elongation. It also prevents the generated FeTiP and boride macromolecular structures from occupying the grain boundaries, forming an isolation layer without forming a connecting bond, and reducing the material's ability to resist low-temperature impact. At the same time, alloys such as phosphorus, manganese, and niobium are added. These elements form connecting bonds at the grain boundaries to strengthen the grain boundary connection. At the same time, the two-phase particles NbCN refine the grains to increase the product strength to more than 350-420 MPa, thereby strengthening the grain boundary's ability to resist low-temperature impact and meeting customer demand for high-strength products with a strength of more than 350-420 MPa.

[0020] (4) The process of the present invention can be industrially applied without other auxiliary equipment and special process equipment, which simplifies the production process and has the characteristics of low cost and high efficiency. At the same time, the present invention has the characteristics of process applicability and strong controllability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the NbCN distribution diagram of the two-phase particles in the steel of Example 1 of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0023] 1. Chemical composition and mechanical properties

[0024] The invention discloses a low-temperature impact-resistant 350MPa-grade ultra-low carbon cold-rolled steel strip, which comprises the following components in mass percentage: C≤0.0020%, Si:≤0.20%, Mn: 0.20%-0.49%, P: 0.020%-0.049%, S≤0.008%, Nb: 0.010%-0.030%, Ti≤0.008%, B≤0.0008%, and the balance is iron and inevitable impurities.

[0025] Specifically, the mechanism of action of each alloy component in the ultra-low carbon cold-rolled steel strip of the present invention is as follows:

[0026] The design of C≤0.002% is conducive to increasing the ductility of the product, carrying out complex deformation applications, improving the R value ≥2.1, and achieving a tensile strength of 350-420MPa. If the C content exceeds the range, it is impossible to form an appropriate amount of NbCN binomial particles and achieve an R value ≥2.1.

[0027] Si: ≤0.20% design is conducive to increasing the ductility of the product, improving the product R value, and increasing the ability to resist low-temperature impact. If the Si content exceeds the range, it is impossible to achieve an R value ≥2.1.

[0028] Mn: The design of 0.20% to 0.49% is conducive to increasing the tensile strength of the product, achieving a tensile strength of more than 350MPa, and increasing the low-temperature impact resistance. If the Mn content exceeds the range, the tensile strength of 350 to 420MPa and the low-temperature impact resistance cannot be achieved.

[0029] P: The design of 0.020% to 0.049% can greatly increase the tensile strength to 350 to 420 MPa. If the P content exceeds the range, the tensile strength of 350 to 420 MPa cannot be achieved.

[0030] S: The design of ≤0.008% can effectively improve the plasticity of the material and prevent hot brittleness. If the S content exceeds the range, the requirements of elongation of 41-52% and R value ≥2.1 cannot be achieved.

[0031] Nb: The design of 0.010% to 0.030% promotes the formation of a large number of fine NbCN two-phase particles in the steel to refine the grains, increase the connectivity of the grain boundaries, increase the low-temperature impact resistance, and increase the yield strength and tensile strength. If the content exceeds the range, the tensile strength of 350 to 420 MPa and the low-temperature impact resistance cannot be achieved.

[0032] Furthermore, the present invention selects Ti≤0.008% and B≤0.0008%, which not only avoids the generation of a large amount of FeTiP by Ti during annealing to reduce the strength and R value of high-strength IF steel, but also avoids the addition of B affecting the yield strength and elongation. At the same time, it can also avoid the generated FeTiP and boride macromolecular structures occupying the grain boundaries, forming an isolation layer without forming a connecting bond, and reducing the material's ability to resist low-temperature impact.

[0033] In a specific embodiment, the tensile strength of the ultra-low carbon cold-rolled steel strip is 350-420 MPa, the yield strength is ≥220-270 MPa, and the elongation is A 50 ≥41%, R value ≥2.1.

[0034] 2. Production technology

[0035] The present invention also discloses a method for preparing a low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip as in any embodiment of the present invention, comprising the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing.

[0036] Further, the method specifically includes the following steps:

[0037] (1) In the pretreatment of hot metal, the hot metal is desulfurized to S≤0.003% to obtain desulfurized hot metal. This is to provide a powerful condition for smelting various auxiliary alloys in the subsequent steelmaking process, thereby controlling the S content required by the finished product.

[0038] (2) During converter smelting, the desulfurized molten iron is smelted in a converter and steel is produced.

[0039] In a specific embodiment, in the converter smelting, the desulfurized molten iron is subjected to converter smelting, and the desulfurized molten iron is subjected to converter smelting. The function is to increase the temperature and reduce the C, P, and S components in the original molten iron, providing a strong condition for refining RH to achieve the smelting components required by the finished product.

[0040] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment.

[0041] In a specific embodiment, the molten steel discharged from the converter is subjected to RH refining treatment, and the outlet temperature is 1586℃~1596℃. The purpose is to treat the molten steel with the smelting composition required by the finished product, and to treat the temperature suitable for molten steel casting, so as to provide a strong condition for the quality of the ingot that meets the customer's requirements. Exceeding the temperature range is likely to cause defects such as increased inclusions.

[0042] (4) In continuous casting, the molten steel after RH refining is continuously cast to obtain continuous casting ingots. The function is to shape the molten steel and produce ingots that meet the quality requirements of customers.

[0043] In a specific embodiment, the baking temperature of the tundish is controlled to be greater than 1100°C, and the superheat of the molten steel in the tundish is 20°C to 40°C; a long water nozzle is used from the ladle to the tundish and argon sealing protection is performed, and the penetration depth of the long water nozzle is 200mm to 400mm; an immersed water nozzle is used from the tundish to the crystallizer and argon sealing protection is performed, and the penetration depth of the crystallizer water nozzle is 100mm to 200mm, and the continuous casting billet is hot delivered.

[0044] (5) During hot rolling, the heating temperature is 1210°C to 1270°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1050°C to 1110°C, the hot rolling final rolling temperature is 900°C to 960°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 660°C to 740°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 7°C / h to 18°C / h to obtain a hot rolled steel coil.

[0045] Specifically, during the hot rolling process, the present invention promotes the precipitation of two-phase particles of reasonable specifications and types through a specific process, and at the same time promotes the formation of a texture that is beneficial to low-temperature impact resistance. Among them, the designed heating temperature is 1210℃~1270℃, the reason is to ensure that the ingot is fully heated and the internal and external temperatures are uniform; the designed rough rolling exit temperature and the finishing rolling start temperature are both 1050℃~1110℃, the reason is that it is conducive to the hot rolling stability of the product; the designed hot rolling final rolling temperature is 900℃~960℃, the reason is to ensure that the product is rolled in the austenite zone; the designed laminar cooling is set to the front-stage concentrated cooling mode, the reason is to increase the grain refinement of the steel plate and improve the low-temperature impact resistance; the designed coiling temperature is 660℃~740℃, the reason is to form a texture that is beneficial to stamping processing, improve the complex processing capability of the product, and at the same time improve the product's resistance to low-temperature impact; after the process coiling, the hot coil is cooled in the form of stacking, and the hot coil cooling rate is designed to be controlled at 7℃ / h~18℃ / h, the reason is to form a grain structure and texture that are beneficial to stamping processing, thereby improving the product's complex processing capability and low-temperature impact resistance.

[0046] S6. In cold rolling, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 0.3 mm to 3.0 mm.

[0047] In a specific embodiment, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5% to 1.0%, and after straightening, it is subjected to shallow groove turbulent pickling with a pickling temperature of 75°C to 85°C and a pickling speed of 80m / min to 300m / min; the pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression rate of ≥50%.

[0048] Specifically, in the cold rolling process stage, the reason for using an elongation of 0.5% to 1.0% is to carry out stretching straightening + shallow groove turbulent pickling to remove the iron oxide scale, so as to achieve a smooth surface of the cold-rolled steel plate, which is beneficial to the customer's subsequent coating and other applications; in addition, the rolling mill rolls the hot coil to the cold-rolled hard steel strip, breaks up the grains, and designs the deformation compression rate to be ≥50%, in order to increase the degree of grain fragmentation of the cold-rolled product, which is beneficial to the recrystallization of the product after annealing to obtain refined grains, and is also more conducive to increasing the grain boundary bonding strength and improving the ability to resist low-temperature impact.

[0049] S6. During annealing, the cold-rolled steel material is continuously annealed to obtain an ultra-low carbon cold-rolled steel strip.

[0050] In a specific embodiment, during annealing, the annealing temperature of the heating section is 750°C to 840°C for the strip, the annealing temperature of the soaking section is 750°C to 840°C for the strip, the slow cooling temperature is 630°C to 680°C, the fast cooling temperature is 350°C to 450°C, the aging section temperature is 350°C to 450°C, and the final cooling temperature is 250°C to 300°C.

[0051] Specifically, in the annealing process, a continuous annealing mode is adopted, and the soaking temperature is set to 750°C to 840°C. The reason is that the precipitated and grown two-phase particles in the steel, such as NbCN, will be dissolved again, and the grains will develop recrystallization texture after rolling. At this time, the elongation and R90 of the steel will increase slightly, while the low-temperature impact resistance will be ensured; the slow cooling temperature is set to 630°C to 680°C, because it is conducive to the precipitation of NbCN as fine two-phase particles, which hinders the development of recrystallization texture. At this time, the yield strength, tensile strength, elongation and low-temperature impact resistance of the steel are ensured; the fast cooling temperature is set to 630°C to 680°C, because it is conducive to the precipitation of NbCN as fine two-phase particles, which hinders the development of recrystallization texture. At this time, the yield strength, tensile strength, elongation and low-temperature impact resistance of the steel are ensured; The cooling temperature is set at 350℃~450℃, because it is to further refine the grains and stabilize the precipitation phase and grain structure; the aging stage temperature is set at 350℃~450℃, because it ensures the precipitation of fine NbCN two-phase particles in the grains, completes the annealing recrystallization transformation of the cold-rolled product, and forms a good 110 texture. Increasing r90 ensures good stamping performance, and does not induce cold brittle FeTiP precipitation, thereby enhancing the ability to resist low-temperature impact; the final cooling temperature is set at 250℃~300℃, because it stabilizes the grain structure and the steel quality no longer changes.

[0052] The following are specific embodiments

[0053] Example 1

[0054] The low-temperature impact resistant 350 MPa grade ultra-low carbon cold-rolled steel strip of this embodiment includes the following components in mass percentage: C: 0.0017%, Si: 0.048%, Mn: 0.49%, P: 0.0456%, S: 0.0052%, Nb: 0.023%, Ti: 0.008%, B: 0.0001%, and the balance is iron and unavoidable impurities.

[0055] The method for preparing the low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment comprises the following steps:

[0056] S1. In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0057] S2. In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0058] In S3 and RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1590°C.

[0059] S4. During continuous casting, the baking temperature of the tundish is controlled to be 1174℃, and the superheat of the molten steel in the tundish is 31℃; a long shroud is used from the ladle to the tundish and argon sealing protection is carried out, and the penetration depth of the long shroud is 267mm; the tundish is covered with a covering agent; an immersed shroud is used from the tundish to the crystallizer and argon sealing protection is adopted, and the penetration depth of the crystallizer shroud is 144mm, and the continuous casting billet is delivered hot.

[0060] S5. During hot rolling, the heating temperature is 1240°C, the rough rolling outlet temperature is 1092°C, the finishing rolling temperature is 1082°C, the hot rolling final rolling temperature is 935°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 700°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 13°C / h to obtain a hot-rolled steel coil.

[0061] S6. During cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5%. After straightening, it is pickled in a shallow groove turbulent flow with a pickling temperature of 81°C and a pickling speed of 285m / min. The pickled steel plate is rolled to a target thickness of 1.2mm for a cold-rolled hard steel strip with a deformation compression ratio of ≥50%.

[0062] S6. During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 788°C for the strip steel, the annealing temperature in the soaking section is 802°C for the strip steel, the slow cooling temperature is 648°C, the fast cooling temperature is 370°C, the aging section temperature is 370°C, and the final cooling temperature is 281°C to obtain an ultra-low carbon cold-rolled steel strip.

[0063] Example 2

[0064] The low-temperature impact resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment includes the following components in mass percentage: C: 0.0016%, Si: 0.047%, Mn: 0.47%, P: 0.041%, S: 0.0064%, Nb: 0.030%, Ti: 0.006%, B: 0.0001%, and the balance is iron and unavoidable impurities.

[0065] The method for preparing the low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment comprises the following steps:

[0066] S1. In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0067] S2. In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0068] In S3 and RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1588°C.

[0069] S4. During continuous casting, the baking temperature of the tundish is controlled at 1301℃, and the superheat of the molten steel in the tundish is 25℃; a long shroud is used from the ladle to the tundish and argon seal protection is carried out, and the penetration depth of the long shroud is 211mm; the tundish is covered with a covering agent; an immersed shroud is used from the tundish to the crystallizer and argon seal protection is adopted, and the penetration depth of the crystallizer shroud is 155mm, and the continuous casting billet is delivered hot.

[0070] S5. During hot rolling, the heating temperature is 1236°C, the rough rolling outlet temperature is 1083°C, the finishing rolling temperature is 1076°C, the hot rolling final rolling temperature is 929°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 709°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 14°C / h to obtain a hot-rolled steel coil.

[0071] S6. During cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5%. After straightening, shallow groove turbulent pickling is performed. The pickling temperature is 83°C and the pickling speed is 267m / min. The pickled steel plate is rolled to a target thickness of 0.7mm for cold-rolled hard steel strip, and the deformation compression rate is ≥50%.

[0072] S6. During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 793°C for the strip steel, the annealing temperature in the soaking section is 807°C for the strip steel, the slow cooling temperature is 652°C, the fast cooling temperature is 408°C, the aging section temperature is 374°C, and the final cooling temperature is 251°C to obtain an ultra-low carbon cold-rolled steel strip.

[0073] Example 3

[0074] The low-temperature impact resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment includes the following components in mass percentage: C: 0.0020%, Si: 0.054%, Mn: 0.44%, P: 0.046%, S: 0.0043%, Nb: 0.028%, Ti: 0.003%, B: 0.0002%, and the balance is iron and unavoidable impurities.

[0075] The method for preparing the low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment comprises the following steps:

[0076] S1. In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0077] S2. In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0078] In S3 and RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1594°C.

[0079] S4. During continuous casting, the baking temperature of the tundish is controlled at 1233℃, and the superheat of the molten steel in the tundish is 28℃; a long shroud is used from the ladle to the tundish and argon sealing protection is carried out, and the penetration depth of the long shroud is 301mm; the tundish is covered with a covering agent; an immersed shroud is used from the tundish to the crystallizer and argon sealing protection is adopted, and the penetration depth of the crystallizer shroud is 177mm, and the continuous casting billet is delivered hot.

[0080] S5. During hot rolling, the heating temperature is 1248°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1104°C, the hot rolling final rolling temperature is 955°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 734°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 15°C / h to obtain a hot-rolled steel coil.

[0081] S6. During cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5%. After straightening, it is pickled in a shallow groove turbulent flow with a pickling temperature of 84°C and a pickling speed of 297m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip to a target thickness of 0.8mm, and the deformation compression rate is ≥50%.

[0082] S6. During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 801°C for the strip steel, the annealing temperature in the soaking section is 823°C for the strip steel, the slow cooling temperature is 655°C, the fast cooling temperature is 354°C, the aging section temperature is 369°C, and the final cooling temperature is 253°C to obtain an ultra-low carbon cold-rolled steel strip.

[0083] Example 4

[0084] The low-temperature impact resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment includes the following components in mass percentage: C: 0.0019%, Si: 0.087%, Mn: 0.42%, P: 0.033%, S: 0.0032%, Nb: 0.026%, Ti: 0.005%, B: 0.0001%, and the balance is iron and unavoidable impurities.

[0085] The method for preparing the low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip of this embodiment comprises the following steps:

[0086] S1. In the pretreatment of molten iron, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

[0087] S2. In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0088] In S3 and RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1595°C.

[0089] S4. During continuous casting, the baking temperature of the tundish is controlled at 1188℃, and the superheat of the molten steel in the tundish is 26℃; a long shroud is used from the ladle to the tundish and argon seal protection is carried out, and the penetration depth of the long shroud is 264mm; the tundish is covered with a covering agent; an immersed shroud is used from the tundish to the crystallizer and argon seal protection is adopted, and the crystallizer penetration shroud depth is 186mm, and the continuous casting billet is delivered hot.

[0090] S5. During hot rolling, the heating temperature is 1241°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1067°C, the hot rolling final rolling temperature is 951°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 721°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 16°C / h to obtain a hot-rolled steel coil.

[0091] S6. During cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5%. After straightening, it is pickled in a shallow groove turbulent flow with a pickling temperature of 84°C and a pickling speed of 297m / min; the pickled steel plate is rolled into a cold-rolled hard steel strip to a target thickness of 1.0mm, and the deformation compression rate is ≥50%.

[0092] S6. During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 829°C for the strip steel, the annealing temperature in the soaking section is 838°C for the strip steel, the slow cooling temperature is 661°C, the fast cooling temperature is 402°C, the aging section temperature is 418°C, and the final cooling temperature is 254°C to obtain an ultra-low carbon cold-rolled steel strip.

[0093] Test Case

[0094] The ultra-low carbon cold-rolled steel strips prepared in Examples 1-4 were subjected to low-temperature impact resistance testing and tensile impact testing. The results are shown in Tables 1 and 2.

[0095] Table 1 Low temperature impact resistance and tensile impact test results of Examples 1-4

[0096]

[0097] According to the results in Table 1, the present invention designs the process coupling technical parameters of the whole process and multiple processes. Through the production process of "molten iron pretreatment + converter smelting + RH refining + continuous casting + hot rolling + cold rolling + annealing" and the alloy composition design, it not only ensures the tensile strength of 350-420MPa, the yield strength of 220-270MPa, and the elongation of A 50 The R value is 41-52%, and it is ≥2.1, while also ensuring the ability to resist low-temperature impact below -80°C.

[0098] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A 350MPa grade ultra-low carbon cold-rolled steel strip resistant to low temperature impact, characterized in that: The following components are included in mass percentage: C≤0.0020%, Si:≤0.20%, Mn:0.20%~0.49%, P:0.020%~0.049%, S≤0.008%, Nb:0.010%~0.030%, Ti≤0.008%, B≤0.0008%, and the balance is iron and inevitable impurities.

2. The low temperature impact resistant 350MPa grade ultra-low carbon cold-rolled steel strip according to claim 1, characterized in that: The ultra-low carbon cold-rolled steel strip has a tensile strength of 350-420 MPa, a yield strength of 220-270 MPa, and an elongation of A 50 It is 41~52%, and the R value is ≥2.

1.

3. A method for preparing a low-temperature impact-resistant 350MPa grade ultra-low carbon cold-rolled steel strip as claimed in any one of claims 1 to 2, characterized in that: The following steps are involved: Hot metal pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing; In the converter smelting, the desulfurized molten iron is subjected to converter smelting and steel is tapped; In the RH refining, the molten steel discharged from the converter is subjected to RH refining treatment; In the continuous casting, the molten steel subjected to the RH refining treatment is continuously cast to obtain a continuously cast ingot; In the hot rolling, the heating temperature is 1210°C to 1270°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1050°C to 1110°C, the hot rolling final rolling temperature is 900°C to 960°C, the laminar cooling is set as a front-stage centralized cooling form, the coiling temperature is 660°C to 740°C, and the hot coil is cooled in the form of stacking after the process coiling, and the hot coil cooling rate is controlled at 7°C / h to 18°C / h to obtain a hot rolled steel coil; In the cold rolling, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 0.3 mm to 3.0 mm; During the annealing, the cold-rolled steel material is continuously annealed to obtain the ultra-low carbon cold-rolled steel strip.

4. The preparation method according to claim 3, characterized in that: In the molten iron pretreatment, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron.

5. The preparation method according to claim 3, characterized in that: In the RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the outlet temperature is 1586°C to 1596°C.

6. The preparation method according to claim 3, characterized in that: In the continuous casting, the baking temperature of the tundish is controlled to be greater than 1100°C, and the superheat of the molten steel in the tundish is 20°C to 40°C; a long nozzle is used from the ladle to the tundish and argon sealing protection is performed, and the penetration depth of the long nozzle is 200mm to 400mm; an immersed nozzle is used from the tundish to the crystallizer and argon sealing protection is performed, and the penetration depth of the crystallizer is 100mm to 200mm, and the continuous casting billet is hot delivered.

7. The preparation method according to claim 3, characterized in that: During the cold rolling, the hot-rolled steel coil is stretched and straightened with an elongation of 0.5% to 1.0%. After straightening, it is subjected to shallow groove turbulent pickling, the pickling temperature is 75° C. to 85° C., and the pickling speed is 230 m / min to 300 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression rate of ≥50%.

8. The preparation method according to claim 3, characterized in that: In the annealing, the annealing temperature of the heating section is 750°C to 840°C for the strip steel, the annealing temperature of the soaking section is 750°C to 840°C for the strip steel, the slow cooling temperature is 630°C to 680°C, the fast cooling temperature is 350°C to 450°C, the aging section temperature is 350°C to 450°C, and the final cooling temperature is 250°C to 300°C.

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