High-strength low-temperature-impact-resistant cold-rolled ultra-low carbon steel and preparation method thereof

Abstract

The invention discloses high-strength low-temperature-impact-resistant cold-rolled ultra-low carbon steel and a preparation method thereof. The high-strength low-temperature-impact-resistant cold-rolled ultra-low carbon steel is prepared from, by mass, 0.0020%-0.0040% of C, smaller than or equal to 0.20% of Si, 0.5%-0.8% of Mn, 0.050%-0.080% of P, smaller than or equal to 0.010% of S, 0.020%-0.050% of Nb, smaller than or equal to 0.008% of Ti, smaller than or equal to 0.0009% of B and the balance iron and inevitable impurities. According to the invention, the process coupling technical parameters of the whole process and multiple procedures are designed, and the production process of molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing is matched with the alloy component design, so that the high strength of 380 MPa is ensured; and the integrity of the product can be still kept when the impact is carried out at the temperature lower than-80 DEG C, and the use in extremely 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 high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel and a preparation method thereof. Background Art

[0002] At present, cold-rolled ultra-low carbon steel is widely used in car bodies, such as 170P1, HC220Y, 250P1, DC04 and DC06. These steel grades are currently 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] After testing, the existing phosphorus-containing ultra-low carbon steel has a low-temperature impact resistance of 0°C, but it is prone to impact brittle fracture below 0°C. This is because the bonding force of the grain boundaries decreases rapidly after being frozen at a low temperature below 0°C, which leads to the limitation of impact fracture at a low temperature below 0°C, causing customers to encounter many low-temperature impact fracture problems during the application process, resulting in the head and tail rolling brittle fracture phenomenon during the cold rolling production process of ultra-low carbon steel in winter, causing a large amount of cold rolling cutting and affecting the subsequent application of automobile OEMs. Therefore, for the production and use of ultra-low carbon steel resistant to low-temperature impact, how to improve the ability of cold-rolled ultra-low carbon steel to resist low-temperature impact has become a crucial research direction, providing solid technical support for the new generation of automotive engineering.

[0004] In the prior art, Chinese patent application number 202010572388.8 discloses a 340MPa-grade strip steel with excellent tensile strength and low-temperature secondary processing performance and a production method, but the product cannot meet the requirements of 380MPa strength products. At the same time, the Ti content of the invention is 0.02% to 0.06%, 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. The B content is 0.0003% to 0.0010%, which will generate more borides. The strong hardenability of boron reduces the elongation of cold plate products. In addition, the minimum R value of the invention is 1.7, which is difficult to meet the car company's requirement for an R value ≥ 2.0, which is not conducive to customers' more complex stamping processing and other applications, and is prone to stamping cracks.

[0005] The Chinese patent application number 202111329449.9 discloses a method for producing a high-strength, low-temperature impact-resistant phosphorus-containing high-strength steel strip, but the product does not have the ability to maintain the integrity of the product when impacted at temperatures below -80°C. At the same time, the Ti content of the invention is 0.02% to 0.05%, which will generate a large amount of FeTiP to reduce the strength and increase the cold brittleness of the steel plate, reducing the ability to resist low-temperature impact. At the same time, the B content is 0.0008% to 0.0020%, which will generate a large amount of borides. The strong hardenability of boron reduces the elongation of cold plate products, and is not conducive to customers' more complex stamping processing and other applications, and is prone to stamping cracks; at the same time, the FeTiP and boride macromolecular structures occupy the grain boundaries to form an isolation layer without forming a connecting bond, so that the molecular activity is low at temperatures below 0°C, the grain boundary connectivity is worse, and low-temperature impact brittle fracture is more likely to occur.

[0006] Chinese patent application number 201410574807.6 discloses a method for producing a low-temperature impact-resistant cold-based structural grade galvanized steel strip. The product of this invention is mainly galvanized products, and the application scenarios are different from those of cold-rolled annealed products. Therefore, the starting point of the composition and process parameter design is inconsistent with the basic mechanism, and the invention does not disclose the mechanical properties of the product. Only from the analysis of the disclosed data, its mechanical properties are not in the same strength level as the present invention; at the same time, the product has a low carbon design C: 0.02%-0.06%, poor processing ductility, and cannot be subjected to complex deformation and deep processing requirements, and does not meet the ability to resist low-temperature impact of -80°C.

[0007] Therefore, for ultra-low carbon steel, the effect of a single control method is not obvious, and it needs to be suppressed based on the full-process process coupling design. Therefore, it is of great significance to explore a full-process multi-dimensional low-temperature impact-resistant preparation method based on composition design, steelmaking, hot rolling, cold rolling, acid rolling, and annealing. Summary of the invention

[0008] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel 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" in combination with the alloy composition design, the cold-rolled products of the ultra-low carbon steel obtained can still maintain the integrity of the product when impacted at 0 degrees to below -80 degrees Celsius, without the phenomenon of impact cold brittle fracture, avoiding the impact cold brittle fracture of various types and components of ultra-low carbon steel cold-rolled products in the past, so that the steel has the ability to resist impact fracture during low-temperature use, and while increasing the processing deformation ability, 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 and strengthen the grain boundary's ability to resist low-temperature impact; in addition, the R value ≥ 2.0 is conducive to the customer's processing of complex deformation deep stamping to produce parts.

[0009] To achieve the above object, 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 in mass percentage: C: 0.0020% to 0.0040%, Si: ≤0.20%, Mn: 0.5% to 0.8%, P: 0.050% to 0.080%, S≤0.010%, Nb: 0.020% to 0.050%, Ti≤0.008%, B≤0.0009%, and the balance is iron and unavoidable impurities.

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

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

[0013] During the converter smelting, the desulfurized molten iron is subjected to converter smelting and steel is tapped;

[0014] In the RH refining, the molten steel discharged from the converter is subjected to RH refining treatment;

[0015] In the continuous casting, the molten steel subjected to the RH refining treatment is continuously cast to obtain a continuously cast ingot;

[0016] In the hot rolling, the heating temperature is 1200°C to 1260°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1040°C to 1080°C, the hot rolling final rolling temperature is 910°C to 970°C, the laminar cooling is set as a front-stage centralized cooling form, the coiling temperature is 680°C to 760°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 10°C / h to 20°C / h to obtain a hot rolled steel coil;

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

[0018] During the annealing, the cold-rolled steel material 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 low-temperature impact resistant 380MPa grade cold-rolled ultra-low carbon steel production process control scheme provided by the present invention breaks the previous status quo of ultra-low carbon steel with poor low-temperature impact resistance ≤0°C, and proposes a full-process multi-step coupling design to improve the low-temperature impact resistance of ultra-low carbon steel, achieving -80°C low-temperature impact without fracture.

[0021] (2) The production process of the present invention does not require additional manufacturing equipment and has the characteristics of low cost and high efficiency. It is also easy to industrialize and mass-produce, has good application effects, and can be industrially applied without the need for other auxiliary equipment and special process equipment, thereby simplifying the production process and reducing production costs.

[0022] (3) The present invention adopts Nb in the form of microalloy two-phase particles to strengthen the low-temperature impact resistance of the product through component design and combined with process designs such as steelmaking process, hot rolling process, acid rolling process, and annealing process, so that the cold-rolled product of the ultra-low carbon steel of the present invention can still maintain the integrity of the product when subjected to impact at 0 degrees to below -80 degrees Celsius, and no cold brittle fracture occurs; at the same time, controlling the Ti content ≤ 0.008% and the B content ≤ 0.0009% can avoid the formation of a large amount of FeTiP precipitates and borides by Ti and B in the previous low-temperature resistant cold-rolled ultra-low carbon steel to weaken the grain boundary bonding force, and avoid the formation of a fracture layer when Ti and B are at ultra-low temperatures. At the same time, after analyzing the design of the cold-rolled ultra-low carbon steel product of the present invention, while increasing the processing deformation capacity, alloys such as phosphorus, manganese, and niobium are added. These elements form strengthening bonds at the grain boundaries to improve the strength. At the same time, a large amount of NbCN fine two-phase particles refine the grains to increase the application strength of the product to more than 380MPa, meeting the customer's application requirements for high-strength products above 380MPa, and improving the impact resistance of the customer's products in low-temperature environments.

[0023] (4) The method for producing ultra-low carbon low-temperature impact-resistant cold-rolled products of the present invention is different from the previous process control of product development. The process parameters of multiple processes in the whole process are designed to achieve the control of the low-temperature impact resistance of cold-rolled ultra-low carbon steel. The cold-rolled ultra-low carbon steel produced by the production method provided by the present invention has a tensile strength of more than 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 38% to 48%; R value ≥ 2.0. At the same time, it will not break at -60℃ impact and -80℃ impact.

[0024] The cold-rolled ultra-low carbon steel not only ensures a high strength level of 380 MPa, but also ensures that the integrity of the product is maintained when impacted at temperatures below -80°C, which is beneficial for use in extremely cold weather in the Arctic and other places; at the same time, the Ti content is ≤0.008%, and the B content is ≤0.0009% which are residual contents, thereby avoiding the formation of FeTiP and borides due to the addition of Ti and B that affect the strength and elongation, and increasing the ability to resist low-temperature impact. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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

[0026] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0027] 1. Chemical composition and mechanical properties

[0028] The invention discloses a high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel, which comprises the following components in mass percentage: C: 0.0020% to 0.0040%, Si: ≤0.20%, Mn: 0.5% to 0.8%, P: 0.050% to 0.080%, S≤0.010%, Nb: 0.020% to 0.050%, Ti≤0.008%, B≤0.0009%, and the balance is iron and inevitable 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] C: 0.0020% to 0.0040%. Ultra-low carbon design is beneficial to increase the ductility of the product, carry out complex deformation applications, and increase the ability to resist low-temperature impact. If the C content exceeds this range, it will not produce an appropriate amount of two-phase particles NbCN, which will affect the product's ability to resist low-temperature impact and the product's processing performance R.

[0031] 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 this range, the content is too much, the lattice is distorted, and the lattice resistance function cannot be completed, which affects the processing ductility of the product.

[0032] Mn: 0.5% to 0.8%, the design is conducive to increasing the tensile strength of the product and increasing the low temperature impact resistance. If the Mn content exceeds this range, the alloy cannot strengthen the ferrite grains and affect the processing performance of the product.

[0033] P: 0.050% to 0.080%, which can significantly increase the tensile strength to more than 380MPa. If the P content exceeds this range, the alloy cannot strengthen the ferrite grains, which affects the processing performance of the product.

[0034] Nb: 0.020% to 0.050%, which promotes the formation of a large number of NbCN fine 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 Nb content exceeds this range, the appropriate amount of two-phase particles NbCN cannot be produced, which affects the low-temperature impact resistance of the product and the processing performance of the product.

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

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

[0037] In a specific embodiment, the cold-rolled ultra-low carbon steel does not break at an impact temperature of -60°C and does not break at an impact temperature of -80°C.

[0038] 2. Production technology

[0039] The present invention also discloses a method for preparing high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel 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, specifically as follows:

[0040] (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.

[0041] (2) During converter smelting, the desulfurized molten iron is smelted in a converter and steel is produced.

[0042] In a specific embodiment, step (2) specifically includes: subjecting the desulfurized molten iron to converter smelting, which serves to increase the temperature and reduce the C, P, and S components in the original molten iron, thereby providing favorable conditions for refining RH to achieve the smelting components required for the finished product.

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

[0044] In a specific embodiment, step (3) specifically includes: subjecting the molten steel discharged from the converter to RH refining treatment, with the RH inlet temperature being 1570°C to 1620°C, the temperature rise being ≤25°C, the static time target being 35min, and the outlet temperature being 1580°C to 1590°C. The purpose is to treat the molten steel with the smelting composition required by the finished product, to treat the temperature suitable for molten steel casting, and 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.

[0045] (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.

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

[0047] (5) During hot rolling, the heating temperature is 1200°C to 1260°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1040°C to 1080°C, the hot rolling final rolling temperature is 910°C to 970°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 680°C to 760°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 10°C / h to 20°C / h to obtain a hot rolled steel coil.

[0048] Specifically, in the hot rolling process stage, a specific process is used to promote the precipitation of two-phase particles of reasonable specifications and types, and at the same time promote the formation of textures that are conducive to low-temperature impact resistance. Among them, the heating temperature is 1200℃~1260℃, the purpose is to ensure that the ingot is fully heated and the internal and external temperatures are uniform; the rough rolling outlet temperature and the finishing rolling start temperature are 1040℃~1080℃, the purpose is to facilitate the hot rolling stability of the product; the hot rolling final rolling temperature is 910℃~970℃, the purpose is to ensure that the product is rolled in the austenite zone, and the laminar cooling is set to the front-stage concentrated cooling mode, the purpose is to increase the grain refinement of the steel plate and improve the low-temperature impact resistance; the coiling temperature is 680℃~760℃, the purpose 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 low-temperature impact resistance; after the process coiling, the hot coil is stacked for cooling, and the hot coil cooling rate is controlled at 10℃ / h~20℃ / h, the purpose is to form a grain structure and texture that is beneficial to stamping processing, thereby improving the product's complex processing capability and low-temperature impact resistance.

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

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

[0051] Specifically, in the cold rolling pickling stage, an elongation of 0.5% to 0.8% is used for stretching straightening + shallow groove turbulent pickling to remove iron oxide scale, so that the cold-rolled steel plate has a smooth surface, which is beneficial to customers' further coating and other applications; the rolling mill rolls the hot coil to the cold-rolled hard steel strip, breaks up the grains, and the deformation compression rate is ≥60%, which increases 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.

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

[0053] In a specific embodiment, step (7) specifically includes: the annealing temperature in the heating section is 760°C to 850°C for the strip, the annealing temperature in the soaking section is 760°C to 850°C for the strip, the slow cooling temperature is 640°C to 690°C, the fast cooling temperature is 450°C to 500°C, the aging section temperature is 440°C to 490°C, and the final cooling temperature is 150°C to 180°C.

[0054] Specifically, the annealing adopts a continuous annealing mode, and the soaking temperature is set at 760℃~850℃, the purpose is to dissolve the precipitated and grown two-phase particles such as NbCN in the steel, and at the same time, the grains develop recrystallization texture after rolling. At this time, the elongation and R90 of the steel are slightly increased while ensuring the low-temperature impact resistance; the slow cooling temperature is set at 640℃~690℃, the purpose is to facilitate the precipitation of NbCN into fine two-phase particles, hindering the development of recrystallization texture. At this time, the yield strength, tensile strength, elongation, and low-temperature impact resistance of the steel are also ensured. impact ability; the rapid cooling temperature is set at 450℃~500℃, the purpose is to further refine the grains, stabilize the precipitation phase and grain structure; the aging stage temperature is set at 440℃~490℃, which can ensure the precipitation of fine NbCN two-phase particles in the grains and complete the annealing recrystallization transformation of the cold-rolled product. Increasing R90 ensures good stamping performance, and at the same time does not cause the precipitation of cold brittle FeTiP, thereby enhancing the ability to resist low-temperature impact; the final cooling temperature is set at 150℃~180℃, the purpose is to stabilize the grain structure, and the steel quality no longer changes.

[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 includes the following components in 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 remainder is iron and unavoidable impurities.

[0058] The preparation method of the high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, which are specifically as follows:

[0059] (1) During 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 subjected to converter smelting.

[0061] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, with the RH inlet temperature being 1580°C, the temperature rise being ≤25°C, the static time target being 35 minutes, and the outlet temperature being 1583°C.

[0062] (4) During continuous casting, the baking temperature of the tundish is controlled to be 1150°C, and the superheat of the molten steel in the tundish is 30°C; a long nozzle is used from the ladle to the tundish and argon sealing protection is provided, and the penetration depth of the long nozzle is 220mmmm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 26 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing protection is provided, and the penetration depth of the crystallizer nozzle is 180mm, and the continuous casting billet is delivered hot.

[0063] (5) During hot rolling, the heating temperature is 1235°C, the rough rolling outlet temperature is 1079°C, the finishing rolling temperature is 1068°C, the hot rolling final rolling temperature is 923°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 701°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 17°C / h to obtain a hot-rolled steel coil.

[0064] (6) During 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 turbulent flow at a temperature of 85° C. and a speed of 200 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥60%.

[0065] (7) During annealing, the cold-rolled steel material 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 rapid 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 includes the following components in 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 is iron and unavoidable impurities.

[0068] The preparation method of the high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, which are specifically as follows:

[0069] (1) During 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 subjected to converter smelting.

[0071] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, with the RH inlet temperature being 1602°C, the temperature rise being ≤25°C, the static time target being 35 minutes, and the outlet temperature being 1581°C.

[0072] (4) During continuous casting, the baking temperature of the tundish is controlled to be 1201°C, and the superheat of the molten steel in the tundish is 30°C; a long nozzle is used from the ladle to the tundish and argon sealing protection is provided, and the penetration depth of the long nozzle is 201 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 25 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing protection is provided, and the penetration depth of the crystallizer nozzle is 149 mm, and the continuous casting billet is delivered hot.

[0073] (5) During hot rolling, the heating temperature is 1243°C, the rough rolling outlet temperature is 1077°C, the finishing rolling start temperature is 1073°C, the hot rolling final rolling temperature is 923°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 702°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 12°C / h to obtain a hot rolled steel coil.

[0074] (6) During 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 turbulent flow with a pickling temperature of 80°C and a pickling speed of 227 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥60%.

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

[0076] Example 3

[0077] The high-strength, low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment includes the following components in 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 is iron and unavoidable impurities.

[0078] The preparation method of the high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, which are specifically as follows:

[0079] (1) During 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 subjected to converter smelting.

[0081] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, with the RH inlet temperature being 1572°C, the temperature rise being ≤25°C, the static time target being 35 minutes, and the outlet temperature being 1589°C.

[0082] (4) During continuous casting, the baking temperature of the tundish is controlled to be 1189°C, and the superheat of the molten steel in the tundish is 33°C; a long nozzle is used from the ladle to the tundish and argon sealing protection is provided, and the penetration depth of the long nozzle is 230 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 27 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing protection is provided, and the penetration depth of the crystallizer nozzle is 177 mm, and the continuous casting billet is delivered hot.

[0083] (5) During hot rolling, the heating temperature is 1245°C, the rough rolling outlet temperature is 1068°C, the finishing rolling start temperature is 1072°C, the hot rolling final rolling temperature is 918°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 698°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 18°C / h to obtain a hot rolled steel coil.

[0084] (6) During 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 turbulent flow with a pickling temperature of 78° C. and a pickling speed of 189 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥60%.

[0085] (7) During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 791°C for the steel strip, the annealing temperature in the soaking section is 805°C for the steel strip, the slow cooling temperature is 652°C, the rapid 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 includes the following components in 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 is iron and unavoidable impurities.

[0088] The preparation method of the high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel of this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, which are specifically as follows:

[0089] (1) During 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 subjected to converter smelting.

[0091] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, with the RH inlet temperature being 1602°C, the temperature rise being ≤25°C, the static time target being 35 minutes, and the outlet temperature being 1588°C.

[0092] (4) During continuous casting, the baking temperature of the tundish is controlled to be 1209°C, and the superheat of the molten steel in the tundish is 27°C; a long nozzle is used from the ladle to the tundish and argon sealing protection is provided, and the penetration depth of the long nozzle is 193 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 32 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing protection is provided, and the penetration depth of the crystallizer nozzle is 172 mm, and the continuous casting billet is delivered hot.

[0093] (5) During hot rolling, the heating temperature was 1256°C, the rough rolling outlet temperature was 1075°C, the finishing rolling start temperature was 1080°C, the hot rolling final rolling temperature was 918°C, the laminar cooling was set as the front-stage concentrated cooling form, the coiling temperature was 709°C, and the hot coil was cooled in the form of stacking after process coiling. The hot coil cooling rate was controlled at 11°C / h to obtain a hot rolled steel coil.

[0094] (6) During 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 turbulent flow with a pickling temperature of 87° C. and a pickling speed of 213 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥60%.

[0095] (7) During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 795°C for the strip steel, the annealing temperature in the soaking section is 809°C for the strip steel, the slow cooling temperature is 655°C, the rapid 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 testing and tensile impact testing. The results are shown in Table 1.

[0098] Table 1 Low temperature impact resistance and tensile impact test results of Examples 1-4

[0099]

[0100] It can be seen from the above embodiments that the production method of the cold-rolled ultra-low carbon steel with a low-temperature impact resistance of 380 MPa of the present invention can greatly improve the quality capability of the cold-rolled ultra-low carbon steel product in terms of low-temperature impact resistance.

[0101] 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 high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel, characterized in that: The following components are included in mass percentage: C: 0.0020% to 0.0040%, Si: ≤0.20%, Mn: 0.5% to 0.8%, P: 0.050% to 0.080%, S≤0.010%, Nb: 0.020% to 0.050%, Ti≤0.008%, B≤0.0009%, and the balance is iron and unavoidable impurities.

2. The high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel according to claim 1, characterized in that: 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 260MPa to 320MPa; The elongation of the cold-rolled ultra-low carbon steel is A 50 38% to 48%; R value ≥ 2.

0.

3. A method for preparing high-strength low-temperature impact-resistant cold-rolled ultra-low carbon steel 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 molten iron pretreatment, the molten iron is desulfurized to S≤0.003% to obtain desulfurized molten iron; During 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 1200°C to 1260°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1040°C to 1080°C, the hot rolling final rolling temperature is 910°C to 970°C, the laminar cooling is set as a front-stage centralized cooling form, the coiling temperature is 680°C to 760°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 10°C / h to 20°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 0.5 mm to 2.0 mm; During the annealing, the cold-rolled steel material is continuously annealed to obtain the cold-rolled ultra-low carbon steel.

4. 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, the RH inlet temperature is 1570°C to 1620°C, the temperature rise range is ≤25°C, the static time target is 35 minutes, and the outlet temperature is 1580°C to 1590°C.

5. 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 1150°C, and the superheat of the molten steel in the tundish is 25°C to 45°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 150mm to 350mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is ≥18 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing protection is performed, and the penetration depth of the crystallizer nozzle is 120mm to 220mm, and the continuous casting billet is hot delivered.

6. 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 0.8%. After straightening, it is subjected to shallow groove turbulent pickling, the pickling temperature is 70°C to 90°C, and the pickling speed is 180m / min to 230m / min; the pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression rate of ≥60%.

7. The preparation method according to claim 3, characterized in that: In the annealing, the annealing temperature of the heating section is 760°C to 850°C for the strip steel, the annealing temperature of the soaking section is 760°C to 850°C for the strip steel, the slow cooling temperature is 640°C to 690°C, the fast cooling temperature is 450°C to 500°C, the aging section temperature is 440°C to 490°C, and the final cooling temperature is 150°C to 180°C.

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