High-toughness cold-rolled ultra-low carbon steel with tensile strength of 440 MPa and preparation method thereof

Abstract

The invention discloses high-toughness cold-rolled ultra-low carbon steel with tensile strength of 440 MPa and a preparation method of the high-toughness cold-rolled ultra-low carbon steel. The high-toughness cold-rolled ultra-low carbon steel comprises the following components in percentage by mass: 0.0030%-0.0060% of C, 0.20%-0.40% of Si, 0.80%-1.50% of Mn, 0.080%-0.095% of P, less than or equal to 0.008% of S, 0.030%-0.080% of Nb, less than or equal to 0.008% of Ti, less than or equal to 0.0010% of B and the balance of iron and inevitable impurities. According to the invention, through component design and combination of processes such as a steelmaking process, a hot rolling process, an acid rolling process and an annealing process, not only can the high strength of 440MPa be ensured, but also the low-temperature impact resistance below-60 DEG C can be ensured, the requirements of customers on application of high-strength products of 440MPa or above can be met, and the impact resistance of the products in a low-temperature environment can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more specifically, to a high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa 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, 210P1, 250P1, DC04 and DC06, etc. These steels are currently widely used in car body covers with complex stamping, such as deep stamping parts such as front cover plates and car dashboards. With the increasing demand for the construction of automobile processing lines in extremely cold areas such as the Arctic and Siberia, the winter temperature is as low as -60℃, so not only does the steel plate need to be stamped, but it also requires strong low-temperature impact resistance, which is a severe challenge for ultra-low carbon steel, especially phosphorus-containing ultra-low carbon steel.

[0003] However, the existing phosphorus-containing ultra-low carbon steel has a low-temperature impact resistance of 0°C. If the temperature is below 0°C, it is prone to impact brittle fracture, which leads to head-to-tail rolling brittle fracture during winter production. This is because the bonding force of the grain boundary 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 many low-temperature impact fracture problems in the application process of customers, resulting in a large amount of cold rolling cutting loss and affecting the subsequent application of automobile OEMs. Therefore, 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 automobile engineering.

[0004] In the prior art, the Chinese patent with application number 202010571885.6 discloses a 440MPa-grade strip steel with excellent secondary processing performance at low temperature and a production method. The plastic strain ratio of the product is ≥1.5r90, which cannot meet the requirements of deep stamping and complex deformation of ultra-low carbon products with 440MPa strength; 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. At the same time, the B content is 0.0005% to 0.0015%, which will generate more borides. The strong hardenability of boron reduces the elongation and n value R value of cold plate products, which is not conducive to customers' more complex stamping processing and other applications, and is prone to stamping cracks; at the same time, the product can only withstand low temperatures of -50°C, which cannot meet the use requirements of extremely low and severe cold areas such as the Arctic and Siberia. The Chinese patent application number 200880011757.3 discloses a hot-dip high-strength steel plate for pressing processing with excellent low-temperature toughness and a manufacturing method thereof. The product can also meet the use requirements of extremely cold areas such as the Arctic and Siberia. At the same time, the Ti content of the product is 0.01% to 0.05%, which will generate a large amount of FeTiP to reduce the strength and increase the cold brittleness of the steel plate, and reduce the low-temperature impact resistance. At the same time, the B content is 0.0005% to 0.0030%, which will generate a large amount of borides. The strong hardenability of boron reduces the elongation and R value of the cold plate product, which is not conducive to customers. Complex stamping processes and other applications can easily cause stamping cracks; at the same time, the FeTiP and boride macromolecular structures occupy the grain boundaries, forming an isolation layer without forming connecting bonds, so that at low temperatures below 0°C, the molecular activity is low and the grain boundary connectivity is worse, and low-temperature impact brittle fracture is more likely to occur; at the same time, the product of this invention is a galvanized product, and its application scenario is different from that of cold-rolled annealed products. Therefore, the starting point of the composition and process parameter design is not consistent with the basic mechanism, and it is not suitable for the use of cold-rolled plates in automobiles. Moreover, the mechanical properties of the products produced by this invention are not disclosed. Only from the analysis of the disclosed data, its mechanical properties cannot meet the requirements. The Chinese patent application number 202010493246.2 discloses an ultra-low carbon bake-hardening steel and a production method thereof. The Nb content in the product composition of the invention is only 0.006-0.010wt%, which cannot form effective NbCN binary particles and does not have the ability to form effective grain boundary low-temperature impact-resistant links; at the same time, it does not have the ability to resist low-temperature impact; in addition, the B content is 0.0005-0.0012wt%, and the product will form borides at 0.0008-0.0012wt%, which will reduce the R value and stamping resistance of the product.

[0005] 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

[0006] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440MPa and a preparation method thereof. The present invention adopts 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, so that the obtained cold-rolled ultra-low carbon steel can still maintain the integrity of the product when impacted at 0 degrees to below -60°C, without the phenomenon of cold brittle fracture, avoiding the impact cold brittle fracture of various types and compositions of ultra-low carbon steel cold-rolled products in the past, thereby having the ability to resist impact fracture during low-temperature use; in addition, R value ≥1.8 can meet the customer's production and application requirements for more complex and deeper stamping; and after analyzing the design of the ultra-low carbon product of the present invention, while increasing the processing deformation capacity, alloys such as phosphorus, manganese, and niobium are added, these elements form effective connecting bonds at the grain boundaries, strengthen the grain boundary connection, and at the same time, the two-phase particles NbCN refine the grains to increase the product strength and strengthen the grain boundary's resistance to low-temperature impact.

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

[0008] A high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa comprises the following components in mass percentage: C: 0.0030% to 0.0060%, Si: 0.20% to 0.40%, Mn: 0.80% to 1.50%, P: 0.080% to 0.095%, S≤0.008%, Nb: 0.030% to 0.080%, Ti≤0.008%, B≤0.0010%, and the balance is iron and unavoidable impurities.

[0009] The present invention also discloses a method for preparing the above-mentioned 440MPa-grade high-toughness cold-rolled ultra-low carbon steel, comprising the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing;

[0010] In the molten iron pretreatment, the molten iron is desulfurized to S≤0.002% to obtain desulfurized molten iron;

[0011] During the converter smelting, the desulfurized molten iron is subjected to converter smelting and steel is tapped;

[0012] In the RH refining, the molten steel discharged from the converter is subjected to RH refining treatment;

[0013] In the continuous casting, the molten steel subjected to the RH refining treatment is continuously cast to obtain a continuously cast ingot;

[0014] In the hot rolling, the heating temperature is 1220°C to 1280°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1060°C to 1120°C, the hot rolling final rolling temperature is 920°C to 980°C, the laminar cooling is set as a front-stage centralized cooling form, the coiling temperature is 600°C to 700°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 5°C / h to 15°C / h to obtain a hot rolled steel coil;

[0015] In the cold rolling, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 0.6 mm to 2.1 mm;

[0016] During the annealing, the cold-rolled steel material is continuously annealed to obtain the cold-rolled ultra-low carbon steel.

[0017] Implementing the embodiments of the present invention will have the following beneficial effects:

[0018] (1) The present invention breaks the current situation of poor low-temperature impact quality of ultra-low carbon steel ≤0°C by designing the composition and combining the process designs of steelmaking process, hot rolling process, acid rolling process, annealing process, etc., and proposes a whole-process multi-process coupling design to improve the low-temperature impact resistance of ultra-low carbon steel. It can not only ensure the high strength of 440MPa, but also ensure the ability to resist low-temperature impact below -60°C. At the same time, the present invention controls the Ti content to ≤0.008% and the B content to ≤0.0010% as residual content, so as to avoid the formation of FeTiP and boride due to the addition of Ti and B, which affects the strength and elongation, and increase the ability to resist low-temperature impact.

[0019] (2) The present invention does not require any additional manufacturing equipment and has the characteristics of low cost and high efficiency. At the same time, the production process of the present invention is easy to industrialize and mass produce, and the application effect is good.

[0020] (3) The composition and process design of the present invention adopt Nb in the form of microalloy two-phase particles to strengthen the low-temperature impact resistance of the product. The cold-rolled ultra-low carbon steel product of the present invention still maintains the integrity of the product when subjected to impact at temperatures ranging from 0 degrees to below -60 degrees Celsius, and no cold brittle fracture occurs. At the same time, the Ti content is set to ≤0.008%, and the B content is set to ≤0.0010%, so as to avoid the use of Ti and B in the previous invention of low-temperature resistant cold-rolled ultra-low carbon steel to generate a large amount of FeTiP precipitates and borides to weaken the grain boundary bonding force, and to avoid the formation of a fracture layer when Ti and B are at ultra-low temperatures. At the same time, the ultra-low carbon product composition design of the present invention increases the processing deformation ability while adding alloys such as phosphorus, manganese, and niobium. 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 above 440MPa, meeting the customer's demand for high-strength products above 440MPa and improving the impact resistance of the product in a low-temperature environment.

[0021] (4) The production method of the present invention is different from the previous process control of product development, and has corresponding measures for low temperature impact resistance. The present invention designs the process parameters of the whole process and multiple steps, and the tensile strength of the cold-rolled ultra-low carbon steel produced is above 440MPa, the yield strength is 250MPa-330MPa, and the elongation is A 50 : 36% ~ 46%, R value ≥ 1.8, at the same time -60 ℃ impact toughness does not break, to achieve a cold-rolled ultra-low carbon steel low temperature impact resistance control, without other auxiliary equipment and special process equipment can be industrialized application, simplify the production process, reduce production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] 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

[0023] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0024] 1. Chemical composition and mechanical properties

[0025] The invention discloses a high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440MPa, comprising the following components in mass percentage: C: 0.0030% to 0.0060%, Si: 0.20% to 0.40%, Mn: 0.80% to 1.50%, P: 0.080% to 0.095%, S≤0.008%, Nb: 0.030% to 0.080%, Ti≤0.008%, B≤0.0010%, and the balance is iron and inevitable impurities.

[0026] Specifically, the mechanism of action of each alloy component in the cold-rolled ultra-low carbon steel of the present invention is as follows:

[0027] C: 0.0030% to 0.0060%. 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.

[0028] Si: 0.20% to 0.40%, which is beneficial to increase the ductility of the product, improve the R value of the product, and increase the ability to resist low-temperature impact. At the same time, Si has strong oxidizability. If the Si content exceeds this range or is too high, thick iron scale will easily form on the surface of the steel plate during hot rolling, affecting the surface quality of the product.

[0029] Mn: 0.80%~1.50 is beneficial to increase the tensile strength of the product and increase 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.

[0030] P: 0.080%~0.095%, can significantly increase the tensile strength to above 440MPa. P content exceeds this range, the alloy cannot strengthen the ferrite grains, affecting the processing performance of the product.

[0031] S: ≤0.008% design can effectively improve the plasticity of the material and prevent hot brittleness. If the S content exceeds the range, the elongation cannot be A. 50 The requirements are 38% to 48% and R value ≥ 2.0.

[0032] Nb: 0.030% to 0.080%, 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.

[0033] Furthermore, the Ti content is designed to be ≤0.008% and the B content is designed to be ≤0.0010% as residual elements to avoid the significant decrease in the strength and r value of the high-strength IF steel due to the generation of a large amount of FeTiP during annealing due to the addition of Ti, to avoid the addition of B affecting the yield strength and elongation, and to also avoid the formation of 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.

[0034] In a specific embodiment, the tensile strength of the cold-rolled ultra-low carbon steel is ≥440MPa, the yield strength is 250MPa-330MPa, and the elongation is A 50 : 36%~46%, R value ≥1.8.

[0035] In a specific embodiment, the cold-rolled ultra-low carbon steel does not break at -60°C impact.

[0036] 2. Production technology

[0037] The present invention also discloses a method for preparing a high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa 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 comprising the following steps:

[0038] (1) In the pretreatment of hot metal, the hot metal is desulfurized to S≤0.002% 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.

[0039] (2) During converter smelting, the desulfurized molten iron is smelted in a converter and steel is produced.

[0040] In a specific embodiment, step (2) specifically includes: subjecting the desulfurized molten iron to converter smelting, and subjecting the desulfurized molten iron to converter smelting. The purpose 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 for the finished product.

[0041] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment.

[0042] In a specific embodiment, step (3) specifically includes: subjecting the molten steel discharged from the converter to RH refining treatment, with the RH outlet temperature being 1589°C to 1599°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.

[0043] (4) During continuous casting, the molten steel that has been subjected to 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.

[0044] (5) In a specific embodiment, step (4) specifically includes: the tundish is controlled to have a baking temperature greater than 1180°C, the tundish molten steel superheat is 15°C to 25°C; a long nozzle is used from the ladle to the tundish and argon sealing is performed, and the penetration depth of the long nozzle is 180mm to 380mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is ≥15 tons; an immersion nozzle is used from the tundish to the crystallizer and argon sealing is performed, and the penetration depth of the crystallizer nozzle is 110mm to 210mm, and the continuous casting billet is hot delivered. The function is to shape the molten steel and produce billets that meet the quality requirements of customers. Exceeding the parameter range is likely to cause defects such as increased inclusions and holes.

[0045] (5) During hot rolling, the heating temperature is 1220°C to 1280°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1060°C to 1120°C, the hot rolling final rolling temperature is 920°C to 980°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 600°C to 700°C, and the hot coil is cooled in the form of stacking after process coiling. The hot coil cooling rate is controlled at 5°C / h to 15°C / h to obtain a hot rolled steel coil.

[0046] 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 a texture that is conducive to low-temperature impact resistance. Among them, the heating temperature is set to 1220℃~1280℃, because it ensures that the billet is fully heated and the internal and external temperatures are uniform; the rough rolling outlet temperature and the finishing rolling start temperature are 1060℃~1120℃, because it is conducive to the stability of hot rolling of the product. The hot rolling final rolling temperature is 920℃~980℃, because it ensures that the product is rolled in the austenite zone, and the laminar cooling is set to the front-stage concentrated cooling mode, because the grain refinement of the steel plate is increased to improve the low-temperature impact resistance; the coiling temperature is 670℃~730℃, because it forms a texture that is beneficial to stamping processing, improves the complex processing capability of the product, and at the same time improves 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 5℃ / h~15℃ / h, because it forms a grain structure and texture that is beneficial to stamping processing, thereby improving the product's complex processing capability and low-temperature impact resistance.

[0047] (6) In the cold rolling, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 0.6 mm to 2.1 mm.

[0048] In a specific embodiment, step (6) specifically includes: stretching and straightening the hot-rolled steel coil with an elongation of 0.5% to 1.0%, and after straightening, performing shallow groove turbulent pickling at a pickling temperature of 75°C to 95°C and a pickling speed of 160m / min to 200m / min; rolling the pickled steel plate into a cold-rolled hard steel strip with a deformation compression ratio of ≥55%.

[0049] Specifically, the reason for using an elongation of 0.5% to 1.0% in the cold rolling and pickling stage is that stretching straightening + shallow groove turbulent pickling are performed to remove the oxide scale, so that the cold-rolled steel plate can achieve a smooth surface, which is beneficial to the customer's further coating and other applications; the rolling mill rolls the hot coil to the cold-rolled hard steel strip, and the grains are broken up. The deformation compression rate ≥55% 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.

[0050] (7) During annealing, the cold-rolled steel material is continuously annealed to obtain cold-rolled ultra-low carbon steel.

[0051] In a specific embodiment, step (7) specifically includes: the annealing temperature in the heating section is 770°C to 860°C for the strip, the annealing temperature in the soaking section is 780°C to 860°C for the strip, the slow cooling temperature is 650°C to 700°C, the fast cooling temperature is 440°C to 480°C, the aging section temperature is 350°C to 450°C, and the final cooling temperature is 130°C to 190°C.

[0052] Specifically, the annealing adopts the continuous annealing mode, and the soaking temperature is set to 780℃~860℃, because 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 650℃~700℃, 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 The rapid cooling temperature is 440℃~480℃, which is to further refine the grains and stabilize the precipitation phase and grain structure; the aging stage temperature is 350℃~450℃, which can ensure the precipitation of fine NbCN two-phase particles in the grains and complete the annealing recrystallization transformation of the cold-rolled products. 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 130℃~190℃, which is to stabilize the grain structure and the steel quality no longer changes.

[0053] The following are specific embodiments

[0054] Example 1

[0055] The high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment includes the following components in mass percentage: C: 0.0055%, Si: 0.26%, Mn: 1.21%, P: 0.087%, S: 0.0056%, Nb: 0.045%, Ti: 0.0008%, B: 0.0008%, and the balance is iron and unavoidable impurities.

[0056] The preparation method of the high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, specifically comprising the following steps:

[0057] (1) During the pretreatment of molten iron, the molten iron is desulfurized to S≤0.002% to obtain desulfurized molten iron.

[0058] (2) In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0059] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1596°C.

[0060] (4) During continuous casting, the baking temperature of the tundish is controlled at 1223°C, and the superheat of the molten steel in the tundish is 23°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 249 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 53 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 173 mm, and the continuous casting billet is delivered hot.

[0061] (5) During hot rolling, the heating temperature is 1245°C, the rough rolling outlet temperature is 1082°C, the finishing rolling start temperature is 1077°C, the hot rolling final rolling temperature is 939°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 673°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.

[0062] (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 with a pickling temperature of 87°C and a pickling speed of 183 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥55% to a target thickness of 1.2 mm.

[0063] (7) During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 804°C, the annealing temperature in the soaking section is 812°C, the slow cooling temperature is 657°C, the rapid cooling temperature is 463°C, the aging section temperature is 408°C, and the final cooling temperature is 141°C.

[0064] Example 2

[0065] The high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment includes the following components in mass percentage: C: 0.0038%, Si: 0.2476%, Mn: 1.014%, P: 0.082%, S: 0.0055%, Nb: 0.0341%, Ti: 0.001%, B: 0.0009%, and the remainder is iron and unavoidable impurities.

[0066] The preparation method of the high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, specifically comprising the following steps:

[0067] (1) During the pretreatment of molten iron, the molten iron is desulfurized to S≤0.002% to obtain desulfurized molten iron.

[0068] (2) In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0069] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1590°C.

[0070] (4) During continuous casting, the baking temperature of the tundish is controlled at 1220°C, and the superheat of the molten steel in the tundish is 22°C; a long nozzle is used from the ladle to the tundish and argon sealing is performed, and the penetration depth of the long nozzle is 221 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 57 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing is performed, and the penetration depth of the crystallizer nozzle is 163 mm, and the continuous casting billet is delivered hot.

[0071] (5) During hot rolling, the heating temperature is 1233°C, the rough rolling outlet temperature is 1078°C, the finishing rolling start temperature is 1073°C, the hot rolling final rolling temperature is 944°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 695°C, and the hot coil is stacked for cooling after the process coiling. The hot coil cooling rate is controlled at 12°C / h to obtain a hot rolled steel coil.

[0072] (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 with a pickling temperature of 85°C and a pickling speed of 186 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression rate of ≥55% to a target thickness of 1.5 mm.

[0073] (7) During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 808°C for the steel strip, the annealing temperature in the soaking section is 818°C for the steel strip, the slow cooling temperature is 653°C, the rapid cooling temperature is 443°C, the aging section temperature is 467°C, and the final cooling temperature is 133°C.

[0074] Example 3

[0075] The high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment includes the following components in mass percentage: C: 0.0055%, Si: 0.252%, Mn: 1.03%, P: 0.083%, S: 0.0055%, Nb: 0.0341%, Ti: 0.005%, B: 0.0003%, and the balance is iron and unavoidable impurities.

[0076] The preparation method of the high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, specifically comprising the following steps:

[0077] (1) During the pretreatment of molten iron, the molten iron is desulfurized to S≤0.002% to obtain desulfurized molten iron.

[0078] (2) In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0079] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1597°C.

[0080] (4) During continuous casting, the baking temperature of the tundish is controlled at 1247°C, and the superheat of the molten steel in the tundish is 21°C; a long nozzle is used from the ladle to the tundish and argon sealing is performed, and the penetration depth of the long nozzle is 264 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 51 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing is performed, and the penetration depth of the crystallizer nozzle is 153 mm, and the continuous casting billet is delivered hot.

[0081] (5) During hot rolling, the heating temperature is 1240°C, the rough rolling outlet temperature is 1084°C, the finishing rolling start temperature is 1081°C, the hot rolling final rolling temperature is 946°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 699°C, and the hot coil is stacked for cooling after the process coiling. The hot coil cooling rate is controlled at 12°C / h to obtain a hot-rolled steel coil.

[0082] (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 84°C and a pickling speed of 194 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥55% to a target thickness of 1.0 mm.

[0083] (7) During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 813°C for the strip steel, the annealing temperature in the soaking section is 811°C for the strip steel, the slow cooling temperature is 672°C, the rapid cooling temperature is 451°C, the aging section temperature is 422°C, and the final cooling temperature is 149°C.

[0084] Example 4

[0085] The high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment includes the following components in mass percentage: C: 0.0055%, Si: 0.251%, Mn: 1.03%, P: 0.0803%, S: 0.0061%, Nb: 0.0342%, Ti: 0.003%, B: 0.0001%, and the balance is iron and unavoidable impurities.

[0086] The preparation method of the high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa in this embodiment comprises the following steps: molten iron pretreatment, converter smelting, RH refining, continuous casting, hot rolling, cold rolling and annealing, specifically comprising the following steps:

[0087] (1) During the pretreatment of molten iron, the molten iron is desulfurized to S≤0.002% to obtain desulfurized molten iron.

[0088] (2) In converter smelting, the desulfurized molten iron is subjected to converter smelting.

[0089] (3) In RH refining, the molten steel discharged from the converter is subjected to RH refining treatment, and the RH outlet temperature is 1598°C.

[0090] (4) During continuous casting, the baking temperature of the tundish is controlled at 1262°C, and the superheat of the molten steel in the tundish is 22°C; a long nozzle is used from the ladle to the tundish and argon sealing is performed, and the penetration depth of the long nozzle is 266 mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is 59 tons; an immersed nozzle is used from the tundish to the crystallizer and argon sealing is performed, and the penetration depth of the crystallizer nozzle is 149 mm, and the continuous casting billet is delivered hot.

[0091] (5) During hot rolling, the heating temperature is 1241°C, the rough rolling outlet temperature is 1089°C, the finishing rolling start temperature is 1083°C, the hot rolling final rolling temperature is 938°C, the laminar cooling is set as the front-stage concentrated cooling form, the coiling temperature is 682°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 obtain a hot rolled steel coil.

[0092] (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 88°C and a pickling speed of 197 m / min. The pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression ratio of ≥55% to a target thickness of 1.0.

[0093] (7) During annealing, the cold-rolled steel material is continuously annealed, the annealing temperature in the heating section is 810°C for the steel strip, the annealing temperature in the soaking section is 815°C for the steel strip, the slow cooling temperature is 674°C, the rapid cooling temperature is 458°C, the aging section temperature is 389°C, and the final cooling temperature is 166°C.

[0094] Test Case

[0095] 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.

[0096] Table 1 Low temperature impact resistance and tensile impact test results of Examples 1-4

[0097]

[0098] It can be seen from the above embodiments that the production method provided by the present invention can greatly improve the quality of cold-rolled ultra-low carbon steel products in terms of their resistance to low-temperature impact.

[0099] 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-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa, characterized in that: The following components are included in mass percentage: C: 0.0030% to 0.0060%, Si: 0.20% to 0.40%, Mn: 0.80% to 1.50%, P: 0.080% to 0.095%, S≤0.008%, Nb: 0.030% to 0.080%, Ti≤0.008%, B≤0.0010%, and the balance is iron and unavoidable impurities.

2. The high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa according to claim 1, characterized in that: The tensile strength of the cold-rolled ultra-low carbon steel is ≥440MPa; The yield strength of the cold-rolled ultra-low carbon steel is 250MPa to 330MPa; The elongation of the cold-rolled ultra-low carbon steel is A 50 : 35%~46%; R value ≥1.

8.

3. A method for preparing a high-toughness cold-rolled ultra-low carbon steel with a tensile strength of 440 MPa 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≤0.002% 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 1220°C to 1280°C, the rough rolling outlet temperature and the finishing rolling start temperature are both 1060°C to 1120°C, the hot rolling final rolling temperature is 920°C to 980°C, the laminar cooling is set as a front-stage centralized cooling form, the coiling temperature is 600°C to 700°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 5°C / h to 15°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 0.6 mm to 2.1 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, and the RH outlet temperature is 1589°C to 1599°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 1180°C, and the superheat of the molten steel in the tundish is 15°C to 25°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 180mm to 380mm; the tundish is covered with a covering agent, and the weight of the molten steel in the tundish is ≥15 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 110mm to 210mm, 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 1.0%. After straightening, it is subjected to shallow groove turbulent pickling, the pickling temperature is 75°C to 95°C, and the pickling speed is 160m / min to 200m / min; the pickled steel plate is rolled into a cold-rolled hard steel strip with a deformation compression rate of ≥55%.

7. The preparation method according to claim 3, characterized in that: In the annealing, the annealing temperature of the heating section is 770°C to 860°C for the strip steel, the annealing temperature of the soaking section is 780°C to 860°C for the strip steel, the slow cooling temperature is 650°C to 700°C, the fast cooling temperature is 440°C to 480°C, the aging section temperature is 350°C to 450°C, and the final cooling temperature is 130°C to 190°C.

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