Highly malleable steel and method for producing the same

Abstract

The present invention provides a high formability steel and a method for manufacturing the same. This steel has the following components, and the mass percentages of the components are: C: 0.10 to 0.35%, Si: 0.8 to 2.0%, Mn: 1.0 to 3.0%, P: ≤0.02%, S ≤0.005%, Al: 0.1 to 2.0%, N: ≤0.005%, and the balance is Fe and other inevitable impurities. The steel of the present invention can achieve a good balance among low yield strength, low yield ratio, high tensile strength, and extremely high elongation, and can be widely applied to components that require complex shapes such as those in commercial vehicles and passenger cars, or other parts that are required to be thinned while maintaining high strength.

Description

Technical Field

[0001] The present invention belongs to the field of steel and its manufacturing method, and particularly relates to high-plasticity steel and its manufacturing method.

Background Art

[0002] Background Currently, as one of the important pillar industries of the national economy, the demand for advanced high-strength steel in the automotive industry is becoming increasingly tight, and the demand for products with low carbon or even no carbon emissions is also rapidly increasing. Making it thinner while maintaining high strength is not only a development trend in the passenger car industry, but also the progress of making it thinner while increasing strength, as well as energy conservation and emissions reduction, which is gradually accelerating in the commercial vehicle field. This is not only a necessity for industrial development, but also an inevitable requirement for the transformation and upgrading of the automotive industry. Especially in the commercial vehicle field, traditional large-tonnage vehicles are increasingly unable to meet the increasingly stringent regulatory requirements. Therefore, designers and manufacturers in the automotive industry are reexamining traditional design and production concepts, aiming for weight reduction while maintaining high strength in various aspects such as chassis, body, and seat, and even using other new materials such as aluminum alloys and carbon fibers. Furthermore, in the commercial vehicle industry, the process of weight reduction is accelerating in terms of chassis, compartment, and upper load. Due to the continuous strengthening of the implementation of commercial vehicle policies and regulations, there is great potential for weight reduction in the future commercial vehicle field.

[0003] Many parts such as the bodies of passenger cars and commercial vehicles, collision beams, fuel tank brackets, battery brackets, gas cylinder brackets, and bent pipes are generally produced from low-strength thick-plate plain steels such as Q235 or Q345, and the process is relatively complex, with some requiring bolt connections and some requiring welding. With the development of lightweighting, many users hope to use cold stamping for the integral forming of these parts of commercial vehicles, which reduces the procedures and achieves lightweighting. As a result, the performance requirements for hot-rolled high-strength steels have increased, and higher elongation and better formability are required while ensuring high strength. When traditional high-strength steels are used for stamping parts such as fuel tank brackets, cracks are likely to occur in the large arc parts of the parts, and smooth stamping cannot be achieved. Therefore, the development of new high-strength steels with high tensile strength and excellent formability is desired. Furthermore, considering the life of the user's molds, the yield strength of the new high-strength steel should not be too high, otherwise, the rebound of the parts will be severe during actual stamping, and forming will become difficult.

[0004] For the reasons described above, the development of hot-rolled high-strength steels or pickled high-strength steels with low yield strength, high tensile strength, and extremely high elongation is desired. This type of steel should be suitable for stamping complex parts with particularly high requirements for cold drawing and forming, have good manufacturability, and can be expected to have a wide range of applications.

[0005] Many applications disclose steels with extremely high plasticity, most of which focus on the field of high-strength cold-rolled steels, while some are related to hot-rolled steels with extremely high plasticity.

[0006] For example, Chinese Patent Application CN104233092A discloses a 780 MPa grade steel with extremely high plasticity. Its composition design has a low carbon content, a high silicon content, and contains a certain amount of precious alloying elements such as Cr, Mo, and Nb, so the alloy cost is relatively high.

[0007] Chinese Patent Application CN107815593A discloses a steel with low silicon content, high aluminum content, and extremely high plasticity. Its composition has a low silicon content, a high aluminum content, and a certain amount of precious element Cu. Its production process mainly includes heat treatment for 1 - 3 minutes in the two-phase region and subsequent phase transformation in the bainite region, and a 780 MPa grade heat-treated steel with extremely high plasticity can be obtained. However, this heat treatment process cannot be applied to existing hot rolling production lines.

Summary of the Invention

[0008] The object of the present invention is to provide a high-plasticity steel and its manufacturing method. This high-plasticity steel has good mechanical properties and can achieve a good balance among low yield strength, low yield ratio, high tensile strength, and extremely high elongation. This steel can be widely applied to components that require complex shapes such as those in commercial vehicles or passenger cars, and other parts that need to be thinned while maintaining high strength.

[0009] To achieve the above object, in a first aspect, the present invention provides a steel having the following components, and the mass percentages of the components are: C: 0.10 - 0.35%, Si: 0.8 - 2.0%, Mn: 1.0 - 3.0%, P: ≦0.02%, S ≦0.005%, Al: 0.1 - 2.0%, N: ≦0.005%, and the balance is Fe and other inevitable impurities.

[0010] Preferably, the above steel also has Ti, and in terms of mass percentage, the content of Ti is 0.2% or less, preferably 0.05 - 0.2%, more preferably 0.05 - 0.1%.

[0011] Preferably, the steel also has one or more selected from the group consisting of Mo, Nb, V, Cu, Ni, Cr, and B; wherein, by mass percentage, the content of Mo is 0.5% or less, preferably 0.3% or less; the content of Nb is 0.1% or less, preferably 0.06% or less; the content of V is 0.1% or less, preferably 0.06% or less; the content of Cu is 0.5% or less, preferably 0.3% or less; the content of Ni is 0.5% or less, preferably 0.3% or less; the content of Cr is 0.5% or less, preferably 0.3% or less; and the content of B is 0.001% or less, preferably 0.0005% or less.

[0012] Preferably, the inevitable impurities in the steel contain, by mass percentage, O of 0.003% or less, preferably O of 0.002% or less; contain S of 0.003% or less; and / or contain N of 0.004% or less.

[0013] Preferably, the mass percentage composition of the steel satisfies one or more of the following items: C is 0.15 - 0.25%, Si is 1.0 - 1.6%, Mn is 1.5 - 2.5%, and Al is 0.3 - 1.0%.

[0014] The design concept of each element in the steel of the present invention is as follows.

[0015] Carbon is a basic element in steel and is also one of the important elements in the present invention. Carbon expands the austenite phase region and stabilizes austenite. As an interstitial atom in steel, carbon plays a very important role in improving the strength of steel and has the greatest influence on the yield strength and tensile strength of steel. Furthermore, as an effective element for stabilizing retained austenite, carbon usually has a relatively high concentration in steel. In the present invention, in order to obtain high-strength steel having different levels of tensile strength and relatively stable retained austenite in the microstructure of the steel, the carbon content must be 0.10% or more. However, the carbon content must not exceed 0.35%. When the carbon content becomes excessive, it is likely to cause high strength, a decrease in elongation, and a decrease in welding performance. Therefore, the carbon content is set to 0.10 to 0.35%.

[0016] Silicon is a basic element in steel and is also one of the important elements in the present invention. When silicon is added to steel, the non-recrystallization temperature of austenite can be decreased and the rolling process window of austenite can be expanded. Thereby, the dynamic recrystallization of steel can be completed during the final rolling stage, which is beneficial for improving the difference between the transverse characteristics and longitudinal characteristics of the steel. Another function of adding silicon to steel is to suppress the formation of cementite. In the present invention, in order to ensure that a large amount of retained austenite is contained in the microstructure of the steel, it is necessary to add a relatively large amount of silicon to suppress the formation of cementite. This carbide formation suppression effect of silicon becomes remarkable when the silicon content is 0.8% or more. However, if the silicon content is made too high, the rolling force load during the actual rolling process becomes too large, and a significant amount of red scale is generated on the steel plate surface, which does not contribute to stable production during rolling. Therefore, the silicon content in the steel is set to 0.8 to 2.0%, preferably 1.0 to 1.6%.

[0017] Manganese is the most fundamental element in steel and is also one of the most important elements in the present invention. Mn expands the austenite phase region, reduces the critical hardening rate of steel, stabilizes austenite, refines grains, and delays the transition from austenite to pearlite. Furthermore, during the heat treatment process, Mn undergoes partitioning and diffuses from bainite to retained austenite, further stabilizing the retained austenite and increasing its content. To achieve these effects, a manganese content of at least 1.0% is required. However, the manganese content should not be too high. If the manganese content exceeds 3.0%, it may lead to segregation in continuous casting slabs and the formation of a large amount of MnS inclusions. Therefore, the manganese content in steel is set to 1.0 - 3.0%, preferably 1.5 - 2.5%.

[0018] Phosphorus is an impurity element in steel. P tends to segregate at grain boundaries. When the P content in steel is relatively high (0.1% or more), Fe2P is formed and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better. Generally, it is desirable to control the P content to 0.02% or less. This is because at this level, it does not increase the steelmaking cost.

[0019] Sulfur is an impurity element in steel. In steel, S typically combines with Mn to form MnS inclusions. Especially when the contents of both S and Mn are relatively high, a significant amount of MnS will be formed in the steel. MnS itself has a certain degree of plasticity. MnS deforms along the rolling direction in the subsequent rolling process, not only reducing the plasticity in the transverse direction of the steel but also increasing the structural anisotropy and adversely affecting the hole expansion performance. Therefore, the lower the S content in steel, the better. To minimize the content of MnS, it is necessary to strictly control the S content. The S content is required to be controlled to 0.005% or less, preferably 0.003% or less.

[0020] Aluminum is one of the important elements in the present invention. In addition to its basic roles of deoxidation and nitrogen fixation, aluminum has two other important functions in the present invention. In the present invention, since the contents of austenite stabilizing elements such as carbon and manganese are relatively high, austenite has strong stability. Therefore, it is difficult to form the required amount of ferrite during the short air cooling stage in the step cooling process after rolling. Therefore, in order to accelerate ferrite transformation and ensure a sufficient amount of ferrite, it is necessary to add more aluminum than that used in conventional high-strength steels. On the other hand, in order to obtain highly stable retained austenite, additional aluminum is also required. Adding aluminum to steel is to accelerate ferrite transformation. Also, during the bainite transformation process, Al not only plays a role in suppressing the formation of cementite, but also promotes the diffusion of carbon atoms from bainite ferrite to retained austenite, thereby accelerating the diffusion of carbon atoms in the retained austenite, increasing the carbon concentration in the retained austenite, and enabling the obtainment of highly stable retained austenite. If the aluminum content is 0.1% or more, various beneficial effects as described above can be achieved. However, when the aluminum content exceeds 2.0%, the effect of promoting carbon diffusion and enrichment is saturated, and the viscosity of the molten steel increases, making the casting nozzle prone to clogging. Therefore, the aluminum content in the steel of the present invention should be 0.1 - 2.0%, preferably 0.3 - 1.0%.

[0021] Nitrogen is an impurity element in the present invention. The lower the nitrogen content, the better. However, nitrogen is an element that cannot be avoided in the steelmaking process. Although in small amounts, nitrogen may combine with strong carbide-forming elements such as Ti to form TiN particles, which is harmful to the performance of the steel. Therefore, in the present invention, the nitrogen content is controlled to be 0.005% or less, preferably 0.004% or less.

[0022] Titanium is one of the optional addition elements in the present invention. Steel with extremely high plasticity and high strength has a large amount of retained austenite, which is a soft phase with a relatively low yield strength. Therefore, in order to improve the yield strength of the steel, fine alloying elements such as titanium can be added under specific conditions. Titanium improves the yield strength due to the precipitation strengthening effect in primary ferrite. With the increase of titanium content, the precipitation strengthening effect gradually increases. When the titanium content reaches 0.20%, the precipitation strengthening effect of titanium saturates. Therefore, the addition amount of titanium can be adjusted as needed. The titanium content in the steel of the present invention is controlled to be 0.20% or less, preferably 0.05 - 0.2%, and more preferably 0.05 - 0.1%.

[0023] Molybdenum is one of the optional addition elements in the present invention. When molybdenum is added to steel, the phase transitions of ferrite and pearlite can be significantly retarded, contributing to obtaining a bainite structure. Furthermore, molybdenum has strong resistance to welding softening. The main objective of the present invention is to obtain a fine structure mainly composed of ferrite, bainite and retained austenite, and since ferrite and bainite are prone to softening after welding, by adding an appropriate amount of molybdenum, the degree of welding softening can be effectively reduced. Considering that molybdenum is a noble metal, if the addition amount is too large, the cost of the alloy will increase. Therefore, the molybdenum content in the present invention is 0.5% or less, preferably 0.3% or less.

[0024] Oxygen is an element that cannot be avoided in the steelmaking process. In the present invention, after deoxidation, the O content in the steel can generally be made 0.003% or less, without having a significant adverse effect on the performance of the steel plate. Therefore, in the present invention, the O content in the steel is controlled to be 0.003% or less, preferably 0.002% or less.

[0025] Copper is one of the optional additive elements in the present invention. When copper is added to steel, the corrosion resistance of the steel can be improved, and when combined with phosphorus, the corrosion resistance effect becomes even better. When the addition amount of Cu exceeds 1%, ε-Cu precipitation phase may be generated under specific conditions, and there is a relatively strong precipitation strengthening effect. However, the addition of Cu is likely to cause the "Cu embrittlement" phenomenon during the rolling process. In order to avoid the significant "Cu embrittlement" phenomenon and fully utilize the corrosion resistance advantages of Cu in specific applications, in the present invention, the content of Cu is controlled to be 0.5% or less, preferably 0.3% or less.

[0026] Nickel is one of the optional additive elements in the present invention. Adding nickel to steel provides a certain degree of corrosion resistance, but its corrosion resistance effect is weaker than that of copper. Adding nickel to steel has little effect on the tensile properties of the steel, but can refine the structure and precipitation phase of the steel and significantly improve the low-temperature toughness of the steel. Furthermore, in the steel added with copper, the occurrence of "Cu brittleness" can be suppressed by adding a small amount of nickel. Even if a relatively large amount of nickel is added, there is no significant adverse effect on the properties of the steel itself. Adding both copper and nickel not only improves the corrosion resistance, but also refines the structure of the steel, and phases precipitate, significantly improving the low-temperature toughness. However, both copper and nickel are relatively expensive alloying elements. Therefore, in order to minimize the cost of alloy design, in the present invention, the addition amount of nickel is 0.5% or less, preferably 0.3% or less.

[0027] Chromium is one of the optional additive elements in the present invention. When chromium is added to steel, its strength can be improved mainly by mechanisms such as solid solution strengthening or refinement of the microstructure. Chromium easily dissolves in ferrite and plays a role in strengthening ferrite. Furthermore, the corrosion resistance can also be improved by adding a small amount of chromium element. Therefore, the addition amount of chromium in the present invention is 0.5% or less, preferably 0.3% or less.

[0028] Niobium is one of the optional addition elements in the present invention. Similar to titanium, niobium is a strong carbide-forming element in steel. When niobium is added to steel, the non-recrystallization temperature of the steel can be significantly increased, enabling the formation of deformed austenite with a higher dislocation density during the final rolling stage, and the final phase transformation structure can be refined during the subsequent transformation process. However, the addition amount of niobium should not be too much. On the other hand, when the addition amount of niobium exceeds 0.01%, relatively coarse niobium-carbonitrides are likely to be formed in the structure, which does not contribute to the low-temperature impact toughness of the steel. On the other hand, a large amount of niobium is likely to cause anisotropy in the hot-rolled austenite structure. Therefore, the niobium content in the steel of the present invention is 0.10% or less, preferably 0.06% or less.

[0029] Vanadium is one of the optional addition elements in the present invention. Similar to Ti and Nb, vanadium is also a strong carbide-forming element. However, vanadium carbide has a low solubility or precipitation temperature and typically dissolves completely in austenite during the final rolling stage. Vanadium carbide begins to form in ferrite only when the temperature drops and the phase transformation starts. Since vanadium carbide has a higher solubility in ferrite compared to niobium carbide and titanium carbide, the size of vanadium carbide formed in ferrite becomes larger, and vanadium carbide is likely to be formed at the grain boundaries, which does not contribute to the toughness of the steel. Therefore, the addition amount of vanadium in the steel of the present invention is 0.10% or less, preferably 0.06% or less.

[0030] Boron is one of the optional additive elements in the present invention. Boron is an element that is prone to segregation. During rolling in the austenite region, the B element segregates to the austenite grain boundaries, reduces the interfacial energy at the austenite grain boundaries, and may inhibit the formation of ferrite during subsequent cooling and phase transformation. Since the desired microstructure of the present invention includes ferrite, bainite, and stable retained austenite, it is necessary to strictly control the content of boron element in the steel to prevent the inhibition of ferrite formation due to excessive addition of boron. Therefore, the boron addition amount in the steel of the present invention is 0.001% or less, preferably 0.0005% or less.

[0031] Unless otherwise specified, the content of elements in the steel of the present invention means mass fraction.

[0032] Preferably, the steel of the present invention has a microstructure including ferrite, bainite, and retained austenite, and the content of retained austenite is 5% or more. Specifically, the volume fraction of ferrite in the steel is 30 - 50%, preferably 35 - 45%; the volume fraction of bainite is 40 - 60%, preferably 45 - 55%; and the volume fraction of retained austenite is 5 - 15%, preferably 10 - 15%.

[0033] Preferably, the above steel has a yield strength of 500 MPa or more, preferably 600 MPa or more, more preferably 700 MPa or more; a tensile strength of 780 MPa or more, preferably 980 MPa or more; and an elongation of 25% or more, preferably 30% or more.

[0034] Preferably, the above steel has a hole expansion rate of 30% or more, preferably 50% or more.

[0035] Preferably, in the steel, when the Ti content is 0.05 to 0.2% and the C content is 0.10 to 0.25%, the yield strength of the steel is 600 MPa or more, the tensile strength is 780 MPa or more, the elongation is 30% or more, and the hole expansion rate is 50% or more; when the Ti content is 0.05 to 0.2% and the C content is 0.25 to 0.35%, the yield strength of the steel is 700 MPa or more, the tensile strength is 980 MPa or more, the elongation is 25% or more, and the hole expansion rate is 30% or more.

[0036] Existing 780 MPa grade steel with extremely high plasticity has C-Si-Mn as the main elements and is refined by adding fine alloying elements such as Nb and Ti as required. In these steels, mainly to utilize the deoxidation and nitrogen fixation functions of the Al element, the Al content is 0.1% or less.

[0037] In contrast, in the present invention, a composition design with a high Al content with the Al content of 0.1% or more is adopted. The main purpose of adding Al at a high content is to promote ferrite transformation and further improve the stability of retained austenite.

[0038] In terms of performance, existing 780 MPa grade steel with extremely high plasticity has a low yield strength or yield ratio and insufficient stability of retained austenite. When deformed, the retained austenite in the microstructure easily transforms into martensite.

[0039] In contrast, the steel of the present invention with extremely high plasticity can have tensile properties in which the yield strength and yield ratio change. Furthermore, the retained austenite in the structure is more stable, and the content of retained austenite is 5% or more. Despite the change in yield strength, the tensile strength and elongation are maintained at a high level, making the steel more preferable for downstream processing and use. Furthermore, the steel of the present invention can also have a relatively high hole expansion rate, making it particularly suitable for stamping processes involving parts with higher requirements for drawing forming and flange forming.

[0040] The above-mentioned method for manufacturing steel has the following steps: 1) Smelting and casting After smelting the components of the above composition in a converter or an electric furnace, secondary refining is carried out in a vacuum furnace, and then casting is performed into a casting blank or a casting ingot; 2) Reheating the casting blank or the casting ingot at a heating temperature of 1100 °C or higher and holding for 1 to 2 hours; 3) Hot rolling and cooling of the casting blank or the casting ingot Here, after hot rolling the casting blank or the casting ingot at an initial rolling temperature of 1000 °C or higher, it is subjected to 5 to 7 passes of rolling at a relatively large deformation rate of 50% or more at 1000 °C or higher. After the intermediate blank reaches 950 °C or higher, it is subjected to 3 to 7 passes of final rolling at a cumulative deformation rate of 70% or more to obtain a steel strip, and the final rolling temperature is 800 to 950 °C; Here, the cooling is step cooling. After final rolling, the steel strip is water-cooled to a temperature of 600 to 750 °C at a cooling rate of 30 °C / s or more, air-cooled for 1 to 10 seconds, then cooled to a temperature of 350 to 550 °C at a cooling rate of 10 °C / s or more and wound into a coil, and then cooled to room temperature at a cooling rate of 50 °C / h or less to obtain a hot-rolled steel strip.

[0041] Furthermore, the above method further includes step 4) pickling. The hot-rolled steel strip is pickled at a running speed of 30 to 120 m / min, a pickling temperature of 75 to 85 °C, and a straightening rate of 2% or less, then washed with water at a temperature in the range of 35 to 50 °C, and the surface of the hot-rolled steel strip is dried at a temperature of 120 to 140 °C and coated with oil to obtain pickled steel with high strength and extremely high plasticity.

[0042] In the method for manufacturing steel of the present invention, The main purpose of setting the initial rolling temperature of hot rolling to 1000 °C or higher and performing 5 to 7 passes of rolling at a relatively large deformation rate of 50% or more at 1000 °C or higher is to refine austenite grains.

[0043] After final rolling in the temperature range of 800 to 950 °C, the contents of ferrite, bainite and retained austenite in the steel are controlled using a step cooling process. The water cooling stop temperature and the air cooling time in the first stage cooling after rolling determine the content of ferrite, and the coil winding temperature after the second stage cooling determines the contents of bainite and retained austenite.

[0044] By combining the step cooling process with an innovative composition design, the contents of ferrite, bainite and retained austenite can be quantitatively controlled. By combining this innovative composition design and process, steel with extremely high plasticity having unusually stable retained austenite can be obtained.

[0045] The innovation of the present invention is as follows.

[0046] The present invention obtains a high-strength steel with extremely high plasticity that is hot-rolled or pickled and has a low yield strength by combining a composition design with medium to low carbon, high silicon, and high aluminum, and an innovative step cooling process, a medium-temperature coil winding process, and a pickling process during hot rolling. A relatively high carbon content is beneficial for obtaining high strength and provides a large number of available carbon atoms that can diffuse into the retained austenite, resulting in very stable retained austenite. The main purpose of adding a relatively high content of silicon is to suppress the formation of carbides and widen the formation temperature range of ferrite. The addition of a relatively high content of aluminum promotes the diffusion of carbon atoms from bainite ferrite to retained austenite and further improves the stability of the retained austenite.

[0047] Preferably, in the present invention, a high content of Ti is added to the steel. By combining this with the step cooling process, nano-sized TiC precipitates are formed in the ferrite grains during the ferrite transformation process, improving the strength of the ferrite, reducing the performance difference between ferrite and bainite, and improving the hole expansion rate of the steel.

[0048] Furthermore, the relatively high manganese content in the steel of the present invention further improves the stability of retained austenite.

[0049] The microstructure of the steel of the present invention, which has a high hole expansion rate and extremely high plasticity, consists of ferrite, bainite and retained austenite. Bainite gives the steel high tensile strength. Ferrite and a relatively high content of metastable retained austenite give the steel sheet extremely high elongation due to the TRIP effect, and the content of retained austenite is 5% or more. Ferrite containing nano-sized precipitates improves the yield strength by precipitation strengthening, reduces the hardness difference between ferrite and bainite, and significantly improves its hole expansion rate while achieving extremely high plasticity. By the precise combination of the above components and processes, an excellent balance of high tensile strength, extremely high elongation, and high hole expansion rate is achieved.

[0050] After hot rolling, the main purpose of coiling at 350 - 550°C is to obtain bainite and retained austenite with high stability. The microstructure of the steel of the present invention with extremely high plasticity mainly consists of ferrite, bainite and stable retained austenite, and the content of retained austenite is 5% or more. Ferrite lowers the yield strength of the steel sheet, bainite increases the tensile strength of the steel sheet, and stable retained austenite makes the elongation of the steel sheet extremely high.

[0051] Based on this innovative composition and process design, the present invention can obtain high-strength steel with low yield strength, extremely high plasticity after hot rolling or pickling. This steel has a yield strength of 500 MPa or more, preferably 600 MPa or more, more preferably 700 MPa or more, and a tensile strength of 780 MPa or more, preferably 980 MPa or more. The elongation of the hot-rolled steel coil or pickled steel coil is 25% or more, preferably 30% or more.

[0052] Compared with the prior art, the advantages of the present invention are as follows.

[0053] Compared with existing steels with extremely high plasticity, the present invention adopts a composition design with medium to low carbon, high silicon, and high aluminum. It is completely different from the traditional designs of existing hot-rolled steels with extremely high plasticity, which have low carbon, high silicon, or low silicon and high aluminum.

[0054] In the steel disclosed in Chinese Patent Application CN104233092A, in addition to low carbon and high silicon, precious metal elements such as Cr, Mo, and Nb are added, so the alloy cost is relatively high. The composition design of Chinese Patent CN107815593A has low silicon and high aluminum and contains a certain amount of Cu. However, its process route mainly involves heat treatment in the two-phase region and phase transformation in the bainite region, and it cannot be applied to the hot-rolling production line.

[0055] Therefore, the above patent applications not only differ from the present invention in terms of composition design but also have problems such as high alloy cost and a process route that is not adaptable to the hot-rolling production line.

[0056] The present invention adopts an innovative composition design concept with medium to low carbon and high aluminum, which is coordinated with an innovative step cooling and medium-temperature coiling process. Using the existing continuous hot-rolling production line, hot-rolled and pickled steels with extremely high plasticity having high tensile strength, extremely high elongation, and high hole expansion ratio can be obtained.

[0057] The steel with high strength and extremely high plasticity manufactured using the technology provided by the present invention has a yield strength of 500 MPa or more, a tensile strength of 780 MPa or more, a low yield ratio, extremely high elongation (A reaches 30% or more), showing an excellent combination of low yield strength, low yield ratio, high tensile strength, extremely high plasticity, and high hole expansion ratio. It is applicable to the manufacture of various complex parts of passenger cars or commercial vehicles and has promising application prospects.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0059] Detailed Description The present invention will be further described below with reference to examples and drawings.

[0060] The compositions of the steels of the examples and comparative examples of the present invention are shown in Table 1. The remainder in the table is Fe and other inevitable impurities.

[0061] The process of the examples of the present invention is as follows. 1) Smelting and casting: According to the composition in Table 1, the molten steel was smelted in a converter or an electric furnace, then subjected to secondary refining in a vacuum furnace, and then cast into a casting blank or a casting ingot. 2) The casting blank or casting ingot was reheated at a heating temperature of 1100 °C or higher and held for 1 to 2 hours. 3) Hot rolling and cooling of casting blank or casting ingot: The casting blank or casting ingot was hot-rolled at an initial rolling temperature of 1000 °C or higher, and then subjected to 5 - 7 passes of rolling with a relatively large deformation rate of 50% or higher at 1000 °C or higher. After the intermediate blank reached 950 °C or higher, it was subjected to final rolling of 3 - 7 passes with a cumulative deformation rate of 70% or higher to obtain a steel strip. The final rolling temperature was 800 - 950 °C.

[0062] The cooling was step cooling. After final rolling, the steel strip was water-cooled to a temperature of 600 - 750 °C at a cooling rate of 30 °C / s or higher. After air-cooling for 1 - 10 seconds, the steel strip was cooled to a temperature of 350 - 550 °C at a cooling rate of 10 °C / s or higher, and after being coiled, it was cooled to room temperature at a cooling rate of 50 °C / h or lower to obtain a hot-rolled steel strip.

[0063] In Examples 1 - 8 and Examples 17 - 24, the pickling process was not carried out, and hot-rolled steel was obtained. In Examples 9 - 16, the pickling process was carried out to obtain pickled steel.

[0064] The specific process of the pickling process is as follows. 4) Pickling: The hot-rolled steel strip was pickled at a running speed of 30 - 120 m / min, a pickling temperature of 75 - 85 °C, and a straightening rate of 2% or lower, then washed with water at a temperature in the range of 35 - 50 °C, and the surface of the hot-rolled steel strip was dried at a temperature of 120 - 140 °C and oil was applied.

[0065] The specific process of the above step 3 is shown in FIGS. 1 and 2.

[0066] The steels of Comparative Examples 1 - 3 are those of CN104233092A.

[0067] Table 2 shows the specific production process parameters of the steels in the examples of the present invention, and does not show the specific process parameters of pickling. Table 3 shows the performance parameters of the hot-rolled steels of Examples 1 - 8 and 17 - 24 of the present invention. Table 4 shows the performance parameters of the pickled steels of Examples 9 - 16 of the present invention.

[0068] The properties of the steels in Tables 3 to 4 were measured as follows.

[0069] The yield strength, tensile strength, and elongation of the steel were tested in accordance with GB / T 228.1-2021 "Tensile Testing of Metallic Materials - Part 1: Test Methods at Room Temperature".

[0070] The hole expansion ratio of the steel was tested in accordance with GB / T 24524-2021 "Metallic Materials, Sheets and Strip, Method of Hole Expansion Test".

[0071] As shown in Table 1, the composition design of the comparative example has low carbon, high silicon, and low aluminum, while the composition design of the examples of the present invention has medium to low carbon, high silicon, and high aluminum. The two have completely different composition designs in terms of carbon content and aluminum content.

[0072] As can be seen from the performance comparison in Table 3, the hole elongation rate of the comparative example is about 20%, while the hole elongation rate of the examples of the present invention reaches about 30%. This shows that the steel with extremely high plasticity of the present invention has an even better balance between strength and extremely high plasticity.

[0073] As can be seen from Tables 3 and 4, the ferrite content in the microstructure of the comparative example is 15% or less, while the ferrite content in the microstructure of the steel of the examples is 25% - 45%. Furthermore, the bainite content in the microstructure of the comparative example is 70% or more, while the bainite content in the microstructure of the examples is 44% - 53%. This shows that there are significant differences in the microstructure designs of the two.

[0074] As can be seen from Tables 3 and 4, the hot-rolled or pickled high-strength steel coils or steel sheets according to the present invention have a yield strength of 500 MPa or more, up to 600 MPa or more, and further up to 700 MPa or more, a tensile strength of 780 MPa or more, up to 980 MPa or more, an elongation of 25% or more, up to 30% or more, and a hole expansion rate of 30% or more, further up to 50% or more. This steel has a good balance of yield strength, tensile strength, extremely high plasticity, and high hole expansion rate, and is particularly suitable for complex formed parts such as automotive chassis structures and cold-drawn parts, and a wide range of applications can be expected.

[0075] Figures 3 to 6 show the metallographic photographs of Examples 1, 6, 10, and 14, respectively. These figures show that the composition and process design according to the present invention achieve a fine structure mainly composed of carbide-free bainite and austenite retained between bainitic laths. Figures 7 to 10 show the typical metallographic photographs of Examples 17, 19, 21, and 23, respectively. These figures clearly show that the composition and process design according to the present invention achieve a fine structure mainly composed of ferrite, bainite, and retained austenite with intragranular nano-precipitates. This fine structure provides a good balance of low yield strength, high tensile strength, extremely high plasticity, and high hole expansion rate, resulting in excellent comprehensive performance. The steel of the present invention has a good balance of strength, extremely high plasticity, and high hole expansion rate, so it is particularly suitable for complex formed parts such as automotive chassis structures, and a wide range of applications can be expected.

[0076]

Table 1

[0077]

Table 2

[0078]

Table 3

[0079]

Table 4

Claims

1. Steel having the following components, wherein the mass percentages of the components are: C: 0.10 to 0.35%, Si: 0.8 to 2.0%, Mn: 1.0 to 3.0%, P: ≤0.02%, S ≤0.005%, Al: 0.1 to 2.0%, N: ≤0.005%, and the balance is Fe and other inevitable impurities, said steel.

2. The steel according to claim 1, wherein the steel further has Ti; in terms of mass percentage, the content of Ti is 0.2% or less, preferably 0.05 to 0.2%, more preferably 0.05 to 0.1%.

3. The steel further has one or more selected from the group consisting of Mo, Nb, V, Cu, Ni, Cr, and B, wherein, in terms of mass percentage, the content of Mo is 0.5% or less, preferably 0.3% or less; the content of Nb is 0.1% or less, preferably 0.06% or less; the content of V is 0.1% or less, preferably 0.06% or less; the content of Cu is 0.5% or less, preferably 0.3% or less; the content of Ni is 0.5% or less, preferably 0.3% or less; the content of Cr is 0.5% or less, preferably 0.3% or less; the content of B is 0.001% or less, preferably 0.0005% or less, the steel according to claim 1 or 2.

4. The steel according to claim 1 or 2, wherein the inevitable impurities contain, in terms of mass percentage, O of 0.003% or less, preferably O of 0.002% or less; S of 0.003% or less; and / or N of 0.004% or less.

5. The steel according to claim 1 or 2, wherein the components of the steel satisfy one or more of the following items, and the items are: in terms of mass percentage, C: 0.15 to 0.25%, Si: 1.0 to 1.6%, Mn: 1.5 to 2.5%, Al: 0.3 to 1.0%.

6. The steel according to any one of claims 1 to 5, wherein the steel has a microstructure of ferrite, bainite, and retained austenite with a content of 5% or more.

7. The steel according to any one of claims 1 to 6, wherein the yield strength is 500 MPa or more, preferably 600 MPa or more, more preferably 700 MPa or more, the tensile strength is 780 MPa or more, preferably 980 MPa or more, and the elongation is 25% or more, preferably 30% or more.

8. In terms of mass percentage, the carbon content is 0.10 to 0.25%, the titanium content is 0.05 to 0.2%, and the yield strength is 600 MPa or more, the tensile strength is 780 MPa or more, the elongation is 30% or more, and the hole expansion rate is 50% or more, or In terms of mass percentage, the carbon content is 0.25 to 0.35%, the titanium content is 0.05 to 0.2%, and the yield strength is 700 MPa or more, the tensile strength is 980 MPa or more, the elongation is 25% or more, and the hole expansion rate is 30% or more, The steel according to claim 2.

9. The steel according to any one of claims 1 to 8, wherein the hole expansion rate of the steel is 30% or more, preferably 50% or more.

10. A method for manufacturing the steel according to any one of claims 1 to 9, including the following steps, wherein the steps are: 1) A smelting and casting step, After smelting the components according to any one of claims 1 to 5 in a converter or an electric furnace, secondary refining is carried out in a vacuum furnace, and then casting is carried out into a casting blank or a casting ingot; 2) A reheating step of the casting blank or the casting ingot, The heating temperature is 1100 °C or more, and the holding time is 1 to 2 hours; 3) A hot rolling and cooling step of the casting blank or the casting ingot, Here, after hot rolling the casting blank or the casting ingot at an initial rolling temperature of 1000 °C or more, it is subjected to 5 to 7 passes of rolling at a relatively large deformation rate of 50% or more at 1000 °C or more, and then, after the intermediate blank reaches 950 °C or more, it is subjected to final rolling of 3 to 7 passes at a cumulative deformation rate of 70% or more to obtain a steel strip; here, the final rolling temperature is 800 to 950 °C; Here, the cooling is step cooling. After final rolling, the steel strip is water-cooled to a temperature of 600 to 750 °C at a cooling rate of 30 °C / s or more, air-cooled for 1 to 10 seconds, then cooled to a temperature of 350 to 550 °C at a cooling rate of 10 °C / s or more, and after being coiled, cooled to room temperature at a cooling rate of 50 °C / h or less to obtain a hot-rolled steel strip. The above method.

11. The method according to claim 10, wherein the method further includes step 4) pickling. The hot-rolled steel strip is pickled at a running speed of 30 to 120 m / min, a pickling temperature of 75 to 85 °C, and a straightening rate of 2% or less, then washed with water at a temperature in the range of 35 to 50 °C, and the surface of the hot-rolled steel strip is dried at a temperature of 120 to 140 °C and oil is applied.

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