Steel with an extremely high hole expansion ratio and method for producing the same

Abstract

The present invention provides a steel with an extremely high hole expansion rate and a method for manufacturing the same. This steel has the following components, and the mass percentages of the components are: C: 0.03 to 0.09%; Si ≤ 0.2%; Mn: 0.5 to 2.0%; P ≤ 0.02%; S ≤ 0.003%; Al: 0.2 to 1.2%; N ≤ 0.004%; Ti ≤ 0.05 to 0.20%; Mo: 0.05 to 0.5%; Mg ≤ 0.005%; O ≤ 0.003%; B ≤ 0.001%; and the balance is Fe and unavoidable impurities, and C, Mn, Mo, and B in the steel satisfy the following formula: 0.25 ≤ 2×C + Mn / 3 + Mo + 150×B ≤ 1.5; where each chemical element represents the numerical value before the percentage symbol of the mass percentage of the corresponding chemical element. The steel according to the present invention is excellent in the harmony of strength, plasticity, and hole expansion performance, and can be applied to automotive chassis parts such as control arms and subframes that require high strength and thinning.

Description

Technical Field

[0001] The present invention relates to steel and a method for manufacturing the same, and particularly to steel having an extremely high hole expansion rate and a method for manufacturing the same.

Background Art

[0002] Background Many parts of passenger cars, particularly chassis and body parts, generally use pickled products. The weight reduction of passenger cars is a development trend in the automotive industry. High strength and weight reduction are inevitable requirements for subsequent new models, which will necessarily lead to higher steel grades and changes in the chassis structure. For example, components will become more complex, and higher requirements will be imposed on material properties and surface and forming technologies such as hydraulic forming, hot stamping, and laser welding. As a result, higher performance will be required for material strength, stamping, flanging, springback, and fatigue characteristics.

[0003] Currently, the high hole expansion rate steels used by domestic automotive component manufacturers are basically high-strength steels with a tensile strength of 600 MPa or less or 400 MPa or less. In China, high hole expansion rate steels with a tensile strength of 780 MPa are gradually beginning to be used in batches. With the increase in the strength of steel, the requirements for two important indicators in the forming process, namely the elongation rate and hole expansion rate of steel, are also becoming higher. In order to further reduce processing costs, some passenger car manufacturers are further increasing the performance requirements for materials. For example, for the control arm for the chassis of a vehicle, which is a structural component, high strength and high plasticity are required. On the other hand, in order to reduce stamping processing and production costs, it is necessary to further improve the hole expansion rate. For example, the hole expansion rate of the current 780 MPa grade high hole expansion rate steel is required to further improve from more than 50% to more than 70%. Currently, most of the 780 MPa grade high hole expansion rate steels adopt the design concept of a high silicon composition system, and the structure is mainly bainite. At the same time, several trace elements are added to the steel to provide a certain precipitation strengthening effect. The surface of the strip steel after pickling not only has obvious red scale, but also the hole expansion rate is basically 50 - 65%, and the elongation rate of the bainite structure is low. None of these can meet the higher hole expansion rate performance requirements proposed by users.

[0004] Regarding the pickled high hole expansion rate steel of 780 MPa grade, there are already some solutions in the prior art. For example:

[0005] Chinese Patent CN103602895A provides a high hole expansion rate steel of low-carbon Nb-Ti microalloy, which adopts a low-carbon and high-silicon composition design in combination with Nb-Ti microalloy and has a hole expansion rate of more than 50%. However, the high-silicon composition design usually results in red scale on the steel plate surface. Also, since the coil winding temperature required for bainite formation is about 500 °C, it is difficult to control the temperature of the entire length of the steel coil, and the performance variation along the entire length is large.

[0006] Chinese Patent CN105821301A provides a hot-rolled high-strength steel with a hole expansion rate of 800 MPa grade and high hole expansion rate. It also adopts a composition design with low carbon and high silicon combined with Nb-Ti microalloy. The Ti content in the steel is very high, ranging from 0.15% to 0.18%. Therefore, in the actual production process, defects such as red scale occur on the surface of the strip steel with such a composition, and the extremely high Ti content tends to form coarse TiN in the steel, which is very harmful to the stability of the hole expansion rate.

[0007] Chinese Patent CN108570604A provides a hot-rolled and pickled steel with a high hole expansion rate of 780 MPa grade. It adopts a composition design with low carbon, high aluminum, and high chromium, and a three-stage cooling process is adopted in the process design. However, this three-stage cooling process is difficult to control, and the actual hole expansion rate of the steel is not high.

[0008] Chinese Patent CN114107792A discloses a hot-rolled and pickled steel with a high hole expansion rate of 780 MPa grade. It adopts a composition design with low carbon and high titanium, and an appropriate amount of Mo is added to the steel. Since the phase transformation process of Mo-containing steel is relatively slow, the phase transformation process mainly occurs after coiling. Therefore, in the actual steel production process, there are problems such as low strength of the inner and outer rings of the steel coil.

Summary of the Invention

[0009] The object of the present invention is to provide a steel with an extremely high hole expansion rate and a manufacturing method thereof. The steel of the present invention has good surface quality, excellent mechanical properties, and is stable. It is suitable for automotive chassis parts such as control arms and subframes that require high strength and thinning.

[0010] Generally, the elongation of a material is inversely proportional to the hole expansion rate, that is, the higher the elongation, the lower the hole expansion rate, and conversely, the lower the elongation, the higher the hole expansion rate. Therefore, it is very difficult to achieve both a high elongation and an extremely high hole expansion rate at the same time, and it is even more difficult to ensure uniform performance over the entire length of the strip steel. For the same or similar strengthening mechanisms, the higher the strength of the material, the lower the hole expansion rate. To obtain steel with good plasticity and combined hole expansion and flanging performance, a better balance between the two is required. To achieve a good harmony among strength, plasticity, and hole expansion performance, it seems essential to add a larger amount of silicon element to steel with high strength, high plasticity, and high hole expansion rate. However, a composition design with a high silicon content usually leads to a deterioration of the surface quality of the steel plate. Specifically, the red scale defects formed during the hot rolling process are difficult to completely remove in the subsequent pickling process. As a result, striped red scales appear on the surface of the pickled high-strength steel, seriously affecting the surface quality.

[0011] To meet the user's requirements for a better balance of higher surface quality, better performance stability, good plasticity, and extremely high hole expansion performance, it is necessary to improve the current pickled steel with a hole expansion rate of 780 MPa grade.

[0012] The present invention adopts a composition design with low silicon (or no Si), low carbon, and high aluminum to avoid the appearance of red scales on the strip steel surface and improve the surface quality of pickled high-strength steel.

[0013] Specifically, the first aspect of the present invention provides steel having the following components, and the mass percentages of the components are: C: 0.03 - 0.09%; Si ≤ 0.2%; Mn: 0.5 - 2.0%; P ≤ 0.02%; S ≤ 0.003%; Al: 0.2 - 1.2%; N ≤ 0.004%; Ti: 0.05 - 0.20%; Mo: 0.05 - 0.5%; Mg ≤ 0.005%; O ≤ 0.003%; B ≤ 0.001%; and the balance is Fe and inevitable impurities. C, Mn, Mo, and B in the steel satisfy the following formula: 0.25 ≤ 2×C + Mn / 3 + Mo + 150×B ≤ 1.5, Here, each chemical element in the formula represents the numerical value before the percent symbol of mass percent. For example, when the C content of the steel is 0.05%, the numerical value 0.05 is substituted for calculation.

[0014] Preferably, 0.25 ≤ 2×C + Mn / 3 + Mo + 150×B ≤ 1.2.

[0015] Unless otherwise specified, the content of a chemical element in the steel means its weight %.

[0016] Preferably, the steel of the present invention further has one or more selected from Nb, V, Cu, Ni, and Cr, and in mass percent, Nb ≤ 0.06%, V ≤ 0.10%, preferably V ≤ 0.05 wt%, Cu ≤ 0.5%, preferably Cu ≤ 0.3 wt%, Ni ≤ 0.5%, preferably Ni ≤ 0.3%, and Cr ≤ 0.5%, preferably Cr ≤ 0.3%.

[0017] Preferably, the components of the steel further satisfy at least one of the following items: Si ≤ 0.15 wt%, Mn: 1.0 - 1.6 wt%, S ≤ 0.0015 wt%, Al: 0.5 - 1.0 wt%, N ≤ 0.003 wt%, Ti: 0.07 - 0.11 wt%, Mo: 0.15 - 0.45 wt%, Ni ≤ 0.03 wt%, B ≤ 0.0005 wt%.

[0018] The design concept of each element in the steel with extremely high hole expansion rate of the present invention is as follows:

[0019] C is a basic element in steel and is one of the important elements in the present invention. C can expand the austenite phase region and stabilize austenite. As an interstitial atom in steel, C plays a very important role in improving the strength of steel, and among them, it has the greatest influence on the yield strength and tensile strength of steel. In the present invention, since the structure to be obtained during the hot rolling stage is close to full ferrite, in order to obtain a high-strength steel with a tensile strength of 780 MPa, the C content needs to be 0.03% or more. When the C content is 0.03% or less, it is difficult for the tensile strength of the ferrite structure to reach 780 MPa, but the C content should be 0.09% or less. If the C content is too high, a pearlite structure is likely to be generated during the phase transformation process, which is harmful to the hole expansion performance. Therefore, the C content needs to be controlled within 0.03 - 0.09%.

[0020] Silicon, which is a basic element in steel, is an impurity element in the present invention. As described above, in order to meet the user requirements of high strength, high plasticity, and extremely high hole expansion rate, a relatively large amount of Si is usually added in the composition design. However, a composition design with a high Si content leads to a deterioration in the surface quality of the steel sheet with more red scale defects. In the present invention, in order to ensure good surface quality, it is necessary to strictly control the Si content. According to a large amount of statistical data in actual production, if the Si content is 0.2% or less, surface red scale defects during the hot rolling process can be avoided. Usually, if the Si content is 0.15% or less, it is guaranteed that no red scale appears. Therefore, the Si content in steel is within 0.2%, preferably within 0.15%.

[0021] Mn is the most fundamental element in steel and also one of the most important elements in the present invention. Mn is an element crucial for expanding the austenite phase region, stabilizing austenite, refining grains, and delaying the transformation from austenite to pearlite. In the present invention, to ensure the strength and grain refinement effect of the steel plate, the Mn content is usually set at 0.5% or more. At the same time, if the Mn content generally does not exceed 2.0%, Mn segregation is likely to occur during steelmaking, and hot cracking is likely to occur during continuous casting of the slab. Therefore, the Mn content in the steel is 0.5 - 2.0%, preferably 1.0 - 1.6%.

[0022] P is an impurity element in steel. P is very likely to segregate at grain boundaries. When the P content in the steel is high (0.1% or more), Fe2P is formed and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower its content, the better. Generally, if the P content is within 0.02%, the performance of the steel will be even better, and the steelmaking cost will not increase.

[0023] S is an impurity element in steel. S in the steel usually combines with Mn to form MnS inclusions. Especially when the contents of both S and Mn are high, more MnS will be formed in the steel. MnS itself has a certain plasticity. During the subsequent rolling process, MnS deforms along the rolling direction, not only reducing the plasticity in the transverse direction of the steel but also increasing the anisotropy of the structure, which is harmful to the hole expansion performance. Therefore, the lower the S content in the steel, the better. To reduce the MnS content, it is necessary to strictly control the S content. In the present invention, the S content is within 0.003%, preferably 0.0015% or less.

[0024] Al is one of the most important elements in the present invention. Adding Al to steel has another important role in the present invention in addition to the conventional deoxidation and nitrogen fixation functions. It is to accelerate the phase transformation process, whereby the phase transformation of the strip is completed on the laminar cooling roller before coiling, and the non-uniform precipitation of nanoscale carbides due to different cooling rates between the inner and outer rings of the steel coil after coiling is avoided, and large performance fluctuations across the entire length of the strip can be avoided. The amount of Al added to the steel is closely related to C and Mn, which are austenite stabilizing elements, and Mo and B, which are the main elements that suppress ferrite phase transformation. Generally, the higher the contents of C, Mg, Mo, and B, the higher the Al content. Therefore, depending on the contents of C, Mg, Mo, and B in the steel, the Al content is usually 0.1 - 1.5%, preferably 0.5 - 1.0%.

[0025] N is an impurity element in the present invention, and the lower its content, the more preferable. However, N is an inevitable element in the steelmaking process. Although its content is relatively low, after being combined with strong carbide-forming elements such as Ti, the resulting TiN particles have an adverse effect on the performance of the steel, especially being very harmful to the hole expansion performance. Due to the square shape of TiN, there is a large stress concentration between its sharp corners and the substrate. During the hole expansion deformation process, the stress concentration between TiN and the substrate easily forms a crack source, resulting in a significant reduction in the hole expansion performance of the material. Since the present invention adopts a design with a high Ti content in the composition system, in order to minimize the adverse effect of TiN on hole expansion, the N content of the present invention is 0.004% or less, preferably 0.003% or less.

[0026] Ti is one of the important elements in the present invention. Ti mainly plays two roles in the present invention: First, it combines with the impurity element N in the steel to form TiN, providing a kind of "nitrogen fixation" effect. Second, it forms fine nano-scale carbides, which are uniformly dispersed in ferrite during the coil winding phase transformation process, improving the strength, plasticity and hole expansion rate. When the Ti content is less than 0.05%, there is no obvious precipitation strengthening effect. When the Ti content is higher than 0.20%, coarse TiN is likely to lead to poor impact toughness of the steel plate. Therefore, the Ti content in the steel of the present invention is 0.05 - 0.20%, preferably 0.07 - 0.11%.

[0027] Mo is one of the important elements in the present invention. Adding Mo to the steel can significantly delay the phase transformation of ferrite and pearlite, contributing to obtaining an irregular ferrite structure. Since Mo and Ti are added to the steel simultaneously and the resulting nano-scale titanium-molybdenum carbide precipitation phase is resistant to coarsening at high temperatures, it ensures that coarsening does not occur over a long period after coil winding and avoids a decrease in strength. At the same time, Mo has strong resistance to welding softening. Since the main purpose of the present invention is to obtain a ferrite + nano-precipitation structure, adding a certain amount of Mo can effectively reduce the level of welding softening. Therefore, the Mo content in the steel of the present invention is 0.1 - 0.5%, preferably 0.15 - 0.45%.

[0028] Mg is one of the important elements in the present invention. Adding Mg in the steel can preferably form fine MgO dispersed during the steelmaking stage. These fine MgO can function as nucleation sites for TiN, effectively increasing the nucleation sites of TiN and reducing the size of TiN in the subsequent continuous casting process. Since TiN has a great influence on the hole expansion rate of the final steel plate, the hole expansion rate is likely to become unstable. Therefore, the Mg content in the steel of the present invention is within 0.005%.

[0029] O is an element that cannot be avoided in the steelmaking process. In the present invention, the O content in the steel can generally be 30 ppm or less after deoxidation, and it will not apparently have an adverse effect on the performance of the steel plate. Therefore, the O content in the steel is within 30 ppm.

[0030] Nb is one of the elements that can be added in the present invention. Nb is a strong carbide element in the steel, similar to Ti. When Nb is added to the steel, it significantly raises the non-recrystallization temperature of the steel, and it is possible to obtain deformed austenite with a higher dislocation density during the final rolling stage, and the final structure can be refined during the subsequent transformation process. However, the amount of Nb should not be too much. On the other hand, when the Nb addition amount exceeds 0.06%, relatively coarse niobium carbonitrides are likely to be generated in the structure, a part of the carbon atoms is consumed, and the precipitation strengthening effect of the carbides decreases. At the same time, when the Nb content is relatively high, anisotropy is likely to occur in the hot-rolled austenite structure, which is inherited by the final structure during the subsequent cooling phase transformation process and is harmful to the hole expansion performance. Therefore, the Nb content in the steel is usually 0.06% or less, preferably 0.03% or less.

[0031] V is an element that can be added in the present invention. Similar to Ti and Nb, V is also a strong carbide-forming element. However, vanadium carbide has a low solution or precipitation temperature and is usually completely dissolved in austenite during the final rolling stage. Only when the temperature drops and the phase transformation begins, V starts to be formed in ferrite. Since the solid solubility rate of vanadium carbide in ferrite is larger than that of Nb and Ti, the size of the vanadium carbide formed in ferrite becomes even larger, which does not contribute to precipitation strengthening and contributes much less to the strength of the steel than titanium carbide or titanium molybdenum carbide. However, since a certain amount of C atoms are consumed for the formation of vanadium carbide, it is harmful to the improvement of the steel strength. Therefore, the V addition amount in the steel is usually 0.10% or less, preferably 0.05% or less.

[0032] Cu is an element that can be added in the present invention. When Cu is added to steel, the corrosion resistance of the steel can be improved. Especially when added together with P element, the corrosion resistance effect will be even better. When the amount of Cu exceeds 1%, an ε-Cu precipitation phase is formed under specific conditions, causing 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 fully exert the corrosion resistance improvement effect of Cu in specific applications without causing a significant "Cu embrittlement" phenomenon, the Cu content is usually within 0.5%, significantly within 0.3%.

[0033] Ni is an element that can be added in the present invention. The addition of Ni to steel has certain corrosion resistance, but it is weaker than that of Cu. The addition of Ni to steel has little effect on the tensile performance of the steel, but it can refine the structure and precipitation phase of the steel, significantly improving the low-temperature toughness of the steel. At the same time, in the steel added with Cu element, the occurrence of "Cu embrittlement" can be suppressed by adding a small amount of Ni. Even if a relatively large amount of Ni is added, there is no significant adverse effect on the performance of the steel itself. If Cu and Ni are added simultaneously, not only can the corrosion resistance be improved, but also the structure and precipitation phase of the steel can be refined, and the low-temperature toughness of the steel can be significantly improved. However, since both Cu and Ni are relatively expensive alloying elements, in order to minimize the cost of alloy design, the addition amount of Ni is usually 0.5% or less, preferably 0.3% or less.

[0034] Cr is an element that can be added in the present invention. Cr is added to steel mainly to improve the strength of the steel by solid solution strengthening or structure refinement. Since the structure of the steel in the present invention is fine bainite ferrite + nano-precipitated carbide, combined with the reduction of mobile dislocations in the structure after the high-temperature bell annealing process, the ratio of the yield strength to the tensile strength of the steel, that is, the yield ratio, is relatively high, usually reaching 0.90 or more. The addition of a small amount of Cr element can appropriately reduce the yield strength of the steel, thereby reducing the yield ratio. Also, the corrosion resistance can be improved by adding a small amount of Cr. The addition amount of Cr is usually 0.5% or less, preferably 0.3% or less.

[0035] B is an impurity element in the present invention. Since B rapidly segregates to austenite grain boundaries during the final rolling stage, it strongly inhibits the ferrite phase transformation. In the present invention, considering that it is expected to obtain a full-ferrite structure as ferrite before hot rolling and coil winding, the content of element B needs to be strictly limited. The addition amount of B to steel is usually 0.001% or less, preferably 0.0005% or less.

[0036] Preferably, the steel of the present invention has a yield strength of 700 MPa or more, a tensile strength of 780 MPa or more, a transverse elongation A50 of 17% or more, and a hole expansion rate of 80% or more.

[0037] Considering the manufacturing cost of steel, preferably, the yield strength of the steel is 850 MPa or less, the tensile strength is 900 MPa or less, the transverse elongation A50 is 25% or less, and the hole expansion rate is 115% or less.

[0038] Preferably, the steel of the present invention has a structure containing 95% by volume or more, preferably 97% by volume or more of ferrite and 5% by volume or less, preferably 3% by volume or less of pearlite, and the ferrite contains dispersed nano-scale carbides.

[0039] Most of the current steels with a high hole expansion rate in the 780 MPa grade adopt a design with a high Ti content in the composition design and simultaneously add alloy elements such as Nb, Mo, and Cr. The structure transformation process mainly occurs after coil winding. Considering that the cooling rates of the inner ring, middle ring, and outer ring of the steel coil after coil winding are different, the strength of the steel coil varies greatly depending on the location. In particular, the inner and outer rings of the steel coil within a certain length range have a significant difference from the performance of the middle ring, resulting in a significant difference in the hole expansion performance of the strip steel.

[0040] In the present invention, in order to improve the uniformity of performance over the entire length of the steel coil, while adding more Al in the composition design, at the same time, by controlling the contents of C, Mn, Mo, and B elements that have an important influence on the ferrite phase transformation, the strip steel can complete the phase transformation process in the air-cooling stage on the laminar flow cooling roller before coil winding, and a strip steel with good structural and precipitation uniformity can be obtained, and the stability of performance can be improved over the entire length of the strip steel.

[0041] Another aspect of the present invention provides a method for manufacturing the above steel, which has the following steps: 1) Smelting and casting; After smelting the molten steel in a converter or an electric furnace according to the above composition, secondary refining is carried out in a vacuum furnace, and then casting is carried out into billets or ingots; 2) Reheating of billets or ingots; The heating temperature is 1200 °C or higher, and the holding time is 1 to 2 hours; 3) Hot rolling and cooling of billets or ingots; Here, the initial rolling temperature is 1050 - 1150 °C. Under high pressure, rough rolling is carried out at 1050 °C or higher for 3 to 5 passes to obtain intermediate billets so that the cumulative deformation rate is 50% or more. Then, the intermediate billets are air-cooled or water-cooled to 950 - 1000 °C, and final rolling is carried out for 5 to 7 passes so that the cumulative deformation rate is 70% or more, and the final rolling temperature is set to 850 - 950 °C to obtain a steel strip; Here, laminar flow cooling is adopted for cooling. After final rolling, the steel strip is water-cooled to 550 - 650 °C at a cooling rate of 10 °C / s or higher, and after coil winding, it is cooled to room temperature at a cooling rate of 50 °C / h or lower to obtain a hot-rolled strip steel.

[0042] Preferably, this method further has step 4) pickling. The pickling operation speed of the hot-rolled strip steel is 30 - 140 m / min, the pickling temperature is 75 - 85 °C, the straightening rate is 3% or less, the water washing is carried out at 35 - 50 °C, and the surface drying and oil coating are carried out at 120 - 140 °C.

[0043] The advantageous effects of the method for manufacturing steel in the present invention are as follows:

[0044] In the present invention, through the high-temperature coil winding process of the hot continuous rolling production line, in order to obtain steel with excellent surface performance and extremely high hole expansion ratio and excellent performance stability, a composition design with specially controlled low carbon and high aluminum is adopted. Due to the innovative design of the composition system, the strip steel can complete the phase transformation before coil winding, avoiding the problem of structural uniformity caused by different cooling rates of the inner ring, middle ring, and outer ring of the steel coil after coil winding, and greatly improving the performance uniformity of the steel coil.

[0045] The present invention adopts Mg deoxidation in the steelmaking process to preferentially generate fine MgO dispersed in the molten steel, creating more nucleation sites for the formation of TiN in the subsequent continuous casting process, effectively refining the TiN particles, and improving the stability of the hole expansion ratio.

[0046] In the present invention, the initial rolling temperature is set at 1050 - 1150°C, and under high pressure, rolling is performed at 1050°C or higher for 3 - 5 passes so that the cumulative deformation rate is 50% or more. The main purpose is to refine austenite grains while retaining more dissolved Ti. Then, the intermediate billet is air-cooled or water-cooled to 950 - 1000°C, and rolling is performed for 5 - 7 passes so that the cumulative deformation rate is 70% or more. After that, after the final rolling between 850 - 950°C, the steel plate is water-cooled to 550 - 650°C at a cooling rate of 10°C / s or higher, and slowly cooled to room temperature after coil winding. The specific manufacturing process is shown in Figure 2.

[0047] During the rough rolling and final rolling stages, the rolling pace should be completed as quickly as possible to ensure that more Ti dissolves into austenite. After final rolling, the strip is cooled inline at a cooling rate of 10 °C / s or more to 550 - 650 °C to obtain a ferrite and nano-precipitation structure. According to actual production experiments, depending on the thickness and composition of the strip, the strip completes all phase transformation processes within 5 - 20 seconds on the laminar cooling roller to obtain a more uniform structure and precipitation.

[0048] In the subsequent pickling process, the inhomogeneity of the thermal stress formed within the steel coil during hot coil winding will be reduced and homogenized during pickling and leveling, which can further improve the uniformity of the steel structure and contribute to obtaining pickled steel with high surface performance, high plasticity, extremely high hole expansion rate, and good performance stability.

[0049] Compared with the prior art, the advantages of the present invention are as follows:

[0050] Compared with the high-silicon composition designs used in Chinese Patents CN103602895A and CN105821301A, the present invention adopts a unique composition design with low silicon, even no silicon, low carbon, and high vanadium to avoid the appearance of red scale on the strip surface and improve the surface quality of pickled high-strength steel.

[0051] The steel of Chinese Patent CN108570604A is designed with a low-silicon composition, and the Si content is 0.05 - 0.5%. However, it is not yet guaranteed to completely remove the red scale defects on the strip surface. Furthermore, the control of the three-stage cooling process is difficult, and it is difficult to ensure the stability of performance.

[0052] In Chinese patents CN105154769A and CN114107792A, since the steel composition contains elements that inhibit ferrite phase transformation such as Mo, a phase transformation process occurs after coiling, and there are problems such as large performance fluctuations between the inner and outer rings of the steel coil during actual production.

[0053] The present invention adopts an innovative composition design with low carbon and high vanadium. By precisely controlling the contents of C, Mn, Mo, and B, a hot-rolled steel coil with high strength, high plasticity, extremely high hole expansion ratio, and performance stability over the entire length can be obtained through a simple rolling process.

[0054] After the pickling process, the internal stress of the ferrite structure is reduced and homogenized. The uniformly refined and dispersed nano-scale carbides in the ferrite, on the one hand, endow the steel plate with high strength and high plasticity, and at the same time, the good structure and uniform distribution of internal stress endow the steel plate with an extremely high hole expansion ratio.

[0055] Using the method of the present invention, it is possible to manufacture steel with an extremely high hole expansion ratio having a yield strength of 700 MPa or more and a tensile strength of 780 MPa or more, while having good elongation (transverse direction A50 ≧ 17%) and high hole expansion performance (hole expansion ratio ≧ 80%). The steel exhibits good performance stability, achieves an excellent harmony of surface performance, strength, plasticity, and hole expansion performance, and is suitable for the manufacture of vehicle chassis, subframes, and other complex parts that require high strength, thinning, and hole expansion flanging.

Brief Description of the Drawings

[0056]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0057] Detailed Description Hereinafter, the present invention will be further described with reference to the attached embodiments and drawings.

[0058] The compositions of the steels of the examples and comparative examples of the present invention are shown in Table 1, where the remainder in Table 1 is Fe and inevitable impurities.

[0059] The process route for manufacturing the steel in the examples of the present invention is as follows: 1) Smelting and casting; After smelting the composition shown in Table 1 in a converter or an electric furnace, secondary refining is performed in a vacuum furnace, and then casting is carried out into billets or ingots; 2) Reheating of billets or ingots; The heating temperature is 1200 °C or higher, and the holding time is 1 to 2 hours; 3) Hot rolling and cooling of billets or ingots; Here, the initial rolling temperature is 1050 - 1150 °C. Under high pressure, rough rolling is performed at 1050 °C or higher for 3 - 5 passes to obtain intermediate billets so that the cumulative deformation rate is 50% or more; then, the intermediate billets are air-cooled or water-cooled to 950 - 1000 °C, and final rolling is performed for 5 - 7 passes so that the cumulative deformation rate is 70% or more, and final rolling is completed at 850 - 950 °C to obtain strip steel; Here, laminar flow cooling is adopted for cooling. The strip steel is water-cooled to 550 - 650 °C at a cooling rate of 10 °C / s or higher, and wound into a coil. After being wound into a coil, it is cooled to room temperature at a cooling rate of 50 °C / h or lower.

[0060] The specific process is shown in Figure 1.

[0061] Table 2 shows the steel manufacturing process parameters of the examples of the present invention. Table 3 shows the performance evaluations of the steels of the examples and comparative examples of the present invention.

[0062] The steels of Comparative Examples 1 to 3 are selected from CN103602895A, and the steel of Comparative Example 4 is selected from CN114107792A.

[0063] Table 1 provides the compositional differences between the examples and the comparative examples. As can be seen from Table 1, all the compositional designs of the comparative examples are designs with low aluminum, and the compositional designs of Comparative Examples 1 to 3 also include designs with high silicon. However, the compositional design of the present invention does not contain silicon and has high aluminum. The compositional designs of the two are completely different.

[0064] As can be seen from Table 3, the steel coil obtained according to the composition and process of the present invention has a yield strength of 700 MPa or more, a tensile strength of 780 MPa or more, a transverse elongation rate A50 of 17% or more, and a hole expansion rate of 80% or more.

[0065] Also, as can be seen from Table 3, Comparative Examples 1 to 3 have the same yield strength, tensile strength, and elongation rate as the present invention, but the hole expansion rates of Comparative Examples 1 to 3 are significantly lower than those of the examples of the present invention.

[0066] The yield strength, tensile strength, and elongation rate of the steels in Table 3 were tested according to GB / T 228.1-2021 "Tensile Testing of Metallic Materials - Part 1: Method of Test at Room Temperature".

[0067] The hole expansion rate of the steel was tested according to GB / T 24524-2021 "Method of Test for Hole Expansion of Thin Sheets and Strips of Metallic Materials".

[0068] Figures 2 to 4 show typical metallographic structure photographs of the steels of Examples 2, 4, and 6 of the present invention, respectively.

[0069] As is clear from the figures, when the composition and process route designed according to the present invention are used, a ferrite-based structure with a very small amount of pearlite can be obtained. Specifically, the ferrite in the steel is 97% by volume or more, the pearlite is 3% by volume or less, and the ferrite contains nano-scale carbides distributed dispersedly.

[0070] The steel of the embodiment of the present invention has a good harmony of high strength, high plasticity and extremely high hole expansion rate, and has excellent comprehensive performance.

[0071] As can be seen from the above embodiments and comparative examples, the 780MPa high-strength steel of the present invention has a good harmony of high strength, high plasticity and extremely high hole expansion rate, and in particular, is suitable for the manufacture of vehicle chassis structures and other parts that require high strength, thinning, hole expansion and flange forming, such as control arms, and can also be used for wheels and other complex parts that require flanging, and its application prospect is broad.

[0072]

Table 1

[0073]

Table 2

[0074]

Table 3

Claims

1. It is a steel having the following components, and the mass percentages of the components are as follows: C: 0.03 to 0.09%; Si ≤ 0.2%; Mn: 0.5 to 2.0%; P ≤ 0.02%; S ≤ 0.003%; Al: 0.2 to 1.2%; N ≤ 0.004%; Ti: 0.05 to 0.20%; Mo: 0.05 to 0.5%; Mg ≤ 0.005%; O ≤ 0.003%; B ≤ 0.001%; and the balance is Fe and inevitable impurities, and C, Mn, Mo and B in the steel satisfy the following formula: 0.25 ≤ 2×C + Mn / 3 + Mo + 150×B ≤ 1.5, wherein each chemical element in the formula represents the numerical value before the percentage symbol of the mass percentage of the corresponding chemical element. The steel described above.

2. It further has one or more elements selected from Nb, V, Cu, Ni and Cr, and in mass percentage, Nb ≤ 0.06%, V ≤ 0.10%, preferably V ≤ 0.05%, Cu ≤ 0.5%, preferably Cu ≤ 0.3 wt%, Ni ≤ 0.5%, preferably Ni ≤ 0.3%, Cr ≤ 0.5%, preferably Cr ≤ 0.3%, and the steel according to Claim 1 is characterized by this.

3. The components of the steel further satisfy at least one of the following items, and the items are: Si ≤ 0.15 wt%, Mn: 1.0 to 1.6 wt%, S ≤ 0.0015 wt%, Al: 0.5 to 1.0 wt%, N ≤ 0.003 wt%, Ti: 0.07 to 0.11 wt%, Mo: 0.15 to 0.45 wt%, Ni ≤ 0.03 wt%, B ≤ 0.0005 wt%, and the steel according to Claim 1 is characterized by this.

4. The steel according to any one of Claims 1 to 3 is characterized by having a yield strength of 700 MPa or more, a tensile strength of 780 MPa or more, a transverse elongation rate A50 of 17% or more, and a hole expansion rate of 80% or more.

5. It has a structure containing 95% by volume or more of ferrite, preferably 97% by volume or more, and containing 5% by volume or less of pearlite, preferably 3% by volume or less, and the ferrite contains nano-scale carbides distributed dispersedly, and the steel according to any one of Claims 1 to 4 is characterized by this.

6. It is a method for manufacturing the steel according to any one of Claims 1 to 5, having the following steps, and the steps are: 1) A smelting and casting step; According to the composition described in any one of claims 1 to 5, after melting steel in a converter or an electric furnace, secondary refining is carried out in a vacuum furnace, and then casting is carried out into billets or ingots; 2) The reheating process of the billets or the ingots; The heating temperature is 1200 °C or higher, and the holding time is 1 to 2 hours; 3) The hot rolling and cooling process of the billets or the ingots; Here, the initial rolling temperature is 1050 - 1150 °C. Under high pressure, rough rolling is carried out at 1050 °C or higher for 3 - 5 passes so that the cumulative deformation rate is 50% or more to obtain intermediate billets. Then, the intermediate billets are air-cooled or water-cooled to 950 - 1000 °C, and final rolling is carried out for 5 - 7 passes so that the cumulative deformation rate is 70% or more, and the final rolling temperature is set to 850 - 950 °C to obtain a steel strip; Here, laminar flow cooling is adopted for cooling. After final rolling, the steel strip is water-cooled to 550 - 650 °C at a cooling rate of 10 °C / s or more, and then wound into a coil. After being wound into a coil, it is cooled to room temperature at a cooling rate of 50 °C / h or less to obtain a hot-rolled steel strip. The above method.

7. The method according to claim 6, further comprising step 4) pickling, where the pickling operation speed of the hot-rolled steel strip is 30 - 140 m / min, the pickling temperature is 75 - 85 °C, the straightening rate is 3% or less, the water washing is carried out at 35 - 50 °C, and the surface drying and oil coating are carried out at 120 - 140 °C.

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