High-hole-expanded steel and method for producing the same

Abstract

The present invention provides a steel and a method for manufacturing the same. The steel contains the following components by mass percentage: C: 0.01 to 0.10%; Si: ≤0.2%; Mn: 0.5 to 2.0%; P: ≤0.02%; S: ≤0.003%; Al: 0.01 to 0.08%; N: ≤0.004%; V: 0.10 to 0.50%; O: ≤0.003%; and the balance is Fe and unavoidable impurities. The steel of the present invention can be applied to automotive chassis parts that require high strength and thinning, such as control arms and subframes.

Description

Technical Field

[0001] The present invention relates to steel and a method for manufacturing the same, particularly high hole-expanding steel and a method for manufacturing the same.

Background Art

[0002] Automobiles play a very important role in the development of the national economy. Many parts of passenger cars, especially parts of the chassis and body, often require hot-rolled pickled products. The lightweighting of passenger cars is a development trend in the automotive industry. To improve the strength and lightweight of automobiles, it is necessary to improve the grade of steel and also improve the structure of the chassis. As a result, the chassis parts become more complex, and the requirements for material properties, surface conditions, and forming technologies such as hydroforming, hot stamping, and laser welding increase. Therefore, performance requirements for materials such as high strength, stamping, flanging, springback, and fatigue performance are required.

[0003] Currently, the high hole-expanding steel used by domestic automotive parts manufacturers is basically high-strength steel with a tensile strength of 600 MPa or less. The competition for high hole-expanding steel with a tensile strength of 540 MPa or less is fierce. At the same time, high hole-expanding steel with a tensile strength of 780 MPa is gradually being used in large quantities in China. However, the requirements for elongation and hole expansion rate, which are two important indicators in the forming process, have also increased, and the requirements for performance stability have become more stringent.

[0004] To reduce process costs, passenger car manufacturers are further improving the performance requirements of materials. For example, when manufacturing automotive chassis parts, in order to shorten the process of the stamping process, the material is required to have high strength and high plasticity, as well as a high hole expansion rate. For example, the hole expansion rate of high hole expansion steel of 780 MPa grade is required to be guaranteed to be 50% or more, preferably further increased to 70% or even 80% or more. Most of the existing high hole expansion steels, especially those of 780 MPa grade, have a hot-rolled bainite structure and are simultaneously strengthened by precipitation phases. Most of the process routes used are medium-temperature coiling, but due to the low temperature control accuracy and poor uniformity of the finished product structure, the properties such as the hole expansion rate of the obtained hot-rolled steel become non-uniform, and stamping cracks are likely to occur during subsequent processing.

[0005] The prior art of pickled high hole expansion steel of 780 MPa grade is listed below.

[0006] Chinese Patent CN103602895A discloses a low-carbon Nb-Ti microalloyed high hole expansion steel, which adopts a composition design combining low carbon and high silicon with Nb-Ti microalloy, whereby a hole expansion rate of 50% or more can be ensured. However, in the design of high silicon composition, usually, red scale occurs on the surface of the steel plate. In addition, since the coiling temperature required to form bainite is usually about 500 °C, it is difficult to control the temperature over the entire length of the steel coil. As a result, the performance of the entire length of the steel coil varies greatly.

[0007] Chinese Patent CN105821301A discloses a hot-rolled high-strength high hole expansion steel of 800 MPa grade, which also adopts a composition design combining low carbon and high silicon with Nb-Ti microalloy. The Ti content of the steel is very high, in the range of 0.15 - 0.18%. In the actual manufacturing process, due to this composition design, defects such as red scale occur on the surface of the strip steel. On the other hand, due to the high Ti content, coarse TiN is likely to be formed in the steel, which has an adverse effect on the stability of the hole expansion rate.

[0008] Chinese Patent CN108570604A discloses a hot-rolled pickled high hole-expansion steel of 780 MPa grade, which adopts a composition design of low carbon, high aluminum, and high chromium, and a three-stage cooling process in the process design. No red scale occurs on the surface of the strip steel, but due to the high aluminum composition design, the casting nozzles are prone to clogging during the actual manufacturing process. On the other hand, the manufacturing process of this steel is complex, especially the three-stage cooling process is difficult to control, resulting in a low hole-expansion rate of the steel.

[0009] Chinese Patent CN114107792A discloses a hot-rolled pickled high hole-expansion steel of 780 MPa grade, which adopts a composition design of low carbon and high titanium, and an appropriate amount of molybdenum is added to the steel. Since the phase transformation process of molybdenum-containing steel is relatively slow, the phase transformation process mainly occurs after coiling. Therefore, in the actual manufacturing process, there are problems such as low strength of the inner and outer rings of the steel coil.

Summary of the Invention

Problems to be Solved by the Invention

[0010] In view of the above-mentioned drawbacks of the prior art, the object of the present invention is to provide a steel and its manufacturing method. The steel of the present invention has high strength, high plasticity, and high hole-expansion rate, and these properties are well coordinated with each other, and can be used for automotive chassis parts such as control arms and subframes that require high strength and thinning.

Means for Solving the Problems

[0011] To achieve the above object, the present invention adopts the following technical solutions. In order to meet the user's requirements for higher surface quality, better performance stability, plasticity, and the coordination of hole-expansion characteristics, it is necessary to improve the conventional high hole-expansion steel.

[0012] Generally, the elongation of a material is inversely proportional to the hole expansion ratio. That is, the higher the elongation, the lower the hole expansion ratio, and conversely, the lower the elongation, the higher the hole expansion ratio. For the same or similar strengthening mechanisms, the higher the strength of the material, the lower the hole expansion ratio. In order to obtain steel with both good plasticity and hole expansion flange workability, it is necessary to better balance the two. To achieve a good harmony among strength, plasticity, and hole expandability, it seems essential to add a larger amount of silicon element to high-strength high-plasticity high-hole-expansion steel. However, the design of high-silicon composition usually leads to a decrease in the surface quality of the steel sheet. Specifically, it is difficult to completely remove the red scale defects formed during the hot rolling process in the subsequent pickling process. As a result, striped red scale appears on the surface of the high-strength steel after pickling, significantly affecting the surface quality.

[0013] In the present invention, a low-carbon high-vanadium element design is adopted without intentionally adding silicon element to the steel. By utilizing nano-sized vanadium carbides formed by adding V element to improve the strength and plasticity of the steel, the chemical composition of the existing hot-rolled steel is optimized. Without changing the existing hot continuous rolling production line, bainite precipitation strengthening type high-strength steel with uniform structure and properties can be obtained.

[0014] Specifically, the steel according to the present invention contains the following components by mass percentage: C: 0.01 - 0.10%; Si ≤ 0.2%; Mn: 0.5 - 2.0%; P ≤ 0.02%; S ≤ 0.003%; Al: 0.01 - 0.08%; N ≤ 0.004%; V: 0.10 - 0.50%; O ≤ 0.003%; and the balance is Fe and unavoidable impurities.

[0015] Preferably, the steel further contains Ti with an upper limit of 0.2%, preferably 0.18%, more preferably 0.015% and a lower limit of 0.05%, preferably 0.08% by mass percentage.

[0016] Preferably, the steel further contains Mo with 0.1 - 0.5%, more preferably 0.20 - 0.40%, still more preferably 0.2 - 0.3% by mass percentage.

[0017] Preferably, the steel further contains one or more components selected from Nb≤0.1%, Cu≤0.5%, Ni≤0.5%, Cr≤0.5%, and B≤0.002%. More preferably, Cu is 0.3% or less; Ni is 0.3% or less; Cr is 0.3% or less; Nb is 0.06% or less; B is 0.002% or less, and even more preferably 0.001% or less.

[0018] Preferably, the composition of the steel satisfies one or more of the following: C: 0.03 to 0.07%; Si≤0.10%; Mn: 0.8 to 1.6%; S≤0.0018%; Al: 0.02 to 0.05%; N≤0.003%; O≤0.002%.

[0019] In the composition design of the steel according to the present invention, C is a basic element in the steel and one of the important elements in the present invention. C can expand the austenite phase region and stabilize austenite. As an interstitial atom in the steel, C plays a very important role in improving the strength of the steel, and among them, it has the greatest influence on the yield strength and tensile strength of the steel. In the present invention, since the structure obtained during the hot rolling stage is low-carbon bainite, in order to obtain a high hole expansion steel with a final tensile strength reaching various strength levels, the C content must be 0.01% or more, but at the same time, the C content should not exceed 0.10%. If the C content is too high, low-carbon martensite is likely to be formed during low-temperature coiling. Therefore, in the present invention, the C content is controlled to be 0.01 to 0.10%, preferably 0.03 to 0.07%.

[0020] Si is a basic element in steel. As described above, in order to meet the user's requirements for high strength, high plasticity, and high hole expansion rate, usually, a larger amount of Si is added in component design. However, the high-silicon component design leads to a decline in the surface quality of the steel plate and an increase in the amount of red scale defects. In the present invention, in order to ensure good surface quality, the Si content should be strictly controlled during component design. In other words, in the present invention, Si is an impurity element. Considering that Si-Mn is required for deoxidation in actual steelmaking, it seems difficult to completely avoid adding Si. According to a large amount of statistical data obtained from actual production, if the Si content is 0.2% or less, surface red scale defects can be avoided during the hot rolling process. Usually, if the Si content is 0.10% or less, it is guaranteed that no red scale appears. Therefore, the Si content in the steel of the present invention is controlled within 0.2%, preferably within 0.10%.

[0021] Mn is also one of the most basic elements in steel and one of the most important elements in the present invention. It is well known that Mn is an important element in expanding the austenite phase region. Mn can reduce the critical hardening rate of steel, stabilize austenite, refine the crystal grains, and delay the transformation from austenite to pearlite. In the present invention, in order to ensure the strength of the steel plate and the crystal grain refinement effect, the Mn content is usually controlled to be 0.5% or more. At the same time, the Mn content generally should not exceed 2.0%. Otherwise, Mn segregation is likely to occur during steelmaking, and hot cracking is also likely to occur during continuous casting of the slab. Therefore, the Mn content in the steel of the present invention is controlled to be 0.5 - 2.0%, preferably 0.8 - 1.6%.

[0022] P is an impurity element in steel. P is very likely to segregate at the grain boundaries. When the P content in the steel is high (0.1% or more), Fe2P is formed and precipitates around the crystal grains, resulting in a decline in the plasticity and toughness of the steel. Therefore, the lower its content, the better. The present invention controls the P content within 0.02%, and the obtained steel has better mechanical properties without increasing the steelmaking cost.

[0023] S is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. Especially when both the contents of S and Mn are high, more MnS is formed in the steel. Since MnS itself has a certain plasticity and can deform along the rolling direction during the subsequent rolling process, not only does the plasticity in the transverse direction of the steel decrease, but the anisotropy of the structure also increases, which has an adverse effect on the hole expansion property. To reduce the MnS content, it is necessary to strictly control the S content. The lower the S content in the steel, the better. In the present invention, the S content is controlled within 0.003%, preferably 0.0018% or less.

[0024] The main functions of Al in steel are deoxidation and nitrogen fixation. When strong carbide-forming elements such as Ti are present, the main functions of Al are deoxidation and grain refinement. In the present invention, Al is a general deoxidizing element and grain-refining element, and its content is usually controlled within 0.01 - 0.08%. However, when the Al content is less than 0.01%, there is no grain refinement effect. Similarly, when the Al content exceeds 0.08%, its grain refinement effect is saturated. Therefore, the Al content in the steel of the present invention is controlled between 0.01 - 0.08%, preferably between 0.02 - 0.05%.

[0025] N is an impurity element in the present invention, and the lower its content, the better. However, N is an inevitable element in the steelmaking process. Although its content is low, when combined with strong carbide-forming elements such as V, the formed VN particles have an adverse effect on the performance of the steel, especially having a very adverse effect on the hole expansion property. Since VN is quadrilateral, a large stress concentration occurs between its acute angle and the base material. During the hole expansion deformation process, a crack source is easily formed due to the stress concentration between VN and the base material, thus significantly reducing the hole expansion property of the material. Since the present invention adopts a high-vanadium design in the composition system, in order to minimize the adverse effect of VN on hole expansion, in the present invention, the N content is controlled within 0.004%, preferably 0.003% or less.

[0026] V is an important element in the present invention. Similar to Ti and Nb, V is also a strong carbide forming element. However, vanadium carbide has a low solution temperature or precipitation temperature and usually completely dissolves in austenite during the finish rolling stage. Only when the temperature decreases and the phase transformation begins, V starts to be formed in ferrite. In order to fully utilize the precipitation strengthening effect of V, the amount of V added to the steel should be at least 0.10% or more so that an obvious precipitation strengthening effect can be obtained. As the V content increases, the precipitation strengthening effect of V gradually increases. When the V content exceeds 0.50%, the precipitation strengthening effect of V saturates, the size of the formed vanadium carbide becomes large, and the contribution to the steel strength decreases conversely. Therefore, the amount of V added to the steel of the present invention is controlled to be 0.50% or less. Specifically, when the V content is 0.10 - 0.20%, a high hole expansion steel of 590 MPa grade can be obtained; when the V content is 0.20 - 0.35%, a high hole expansion steel of 780 MPa grade can be obtained; when the V content is 0.35 - 0.50%, a high hole expansion steel of 980 MPa grade can be obtained.

[0027] Mo is one of the important elements in the present invention. When Mo is added to the steel, it can significantly delay the phase transformation of ferrite and pearlite, which is beneficial for obtaining a bainite structure. In addition, Mo has strong resistance to welding softening. The main purpose of the present invention is to obtain a low-carbon bainite structure. However, low-carbon bainite tends to soften after welding. Therefore, by adding a certain amount of Mo, the degree of welding softening can be effectively reduced. Therefore, in the present invention, the Mo content is controlled to be 0.10 - 0.50%, preferably 0.20 - 0.40%, more preferably 0.2 - 0.3%. Combined with the step cooling process, Mo plays a certain inhibitory role in the formation of ferrite during the step cooling process. If the Mo content is within the above range, its effect can be fully realized.

[0028] Nb is one of the elements that can be added in the present invention. Similar to Ti, Nb is a strong carbide element in steel. When Nb is added to steel, the non-recrystallization temperature of the steel rises significantly, and deformed austenite with a higher dislocation density can be obtained in the finish rolling stage, and the final phase transformation structure can be refined during the subsequent transformation process. However, the addition amount of Nb should not be too much. On the other hand, when the addition amount of Nb exceeds 0.10%, relatively coarse niobium carbonitrides are likely to be formed in the structure, and part of the carbon atoms are consumed, resulting in a decrease in the precipitation strengthening effect of carbides. At the same time, when the Nb content is high, anisotropy of the hot-rolled austenite structure also tends to occur, which is inherited to the final structure during the subsequent cooling phase transformation process, and the hole expansion property is impaired. Therefore, the Nb content in the steel of the present invention is controlled to be 0.10% or less, preferably 0.06% or less.

[0029] Ti is an optional element in the present invention. When a small amount of Ti is added to steel, on the one hand, it can combine with N to form TiN during the high-temperature stage, fix N and help reduce the formation of subsequent VN; on the other hand, the excess Ti after combining with N can combine with C during the subsequent process to form nano-TiC, and together with nano-VC, the performance of the steel can be improved. When the Ti content exceeds 0.20%, coarse TiN is likely to be formed during the high-temperature stage, and the impact toughness of the steel deteriorates. Therefore, the content of Ti, which is an optional element in the steel of the present invention, is within 0.20%, preferably within 0.18%, more preferably within 0.015%, and most preferably within 0.10%. On the other hand, the Ti content is preferably 0.05% or more, more preferably 0.08% or more, whereby an excellent precipitation strengthening effect can be obtained.

[0030] Cu is an optional element in the present invention. When Cu is added to steel, the corrosion resistance of the steel can be improved. When added together with the P element, the corrosion resistance performance becomes better. However, when the addition amount of Cu exceeds 1%, an ε-Cu precipitation phase may be formed under certain conditions, resulting in a strong precipitation strengthening effect. However, when Cu is added, the "Cu brittleness" phenomenon is likely to occur during the rolling process. In order to fully utilize the corrosion resistance improvement effect of Cu in a specific application without causing a significant "Cu embrittlement" phenomenon, in the present invention, the Cu content is controlled within 0.5%, preferably within 0.3%.

[0031] Ni is an optional element in the present invention. When Ni is added to steel, a certain degree of corrosion resistance is shown, but the corrosion resistance effect is weaker than that of Cu. Adding 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, adding a small amount of Ni to the steel containing the Cu element can suppress the occurrence of "Cu brittleness". Adding more Ni will not have a significant adverse effect on the performance of the steel itself. Adding Cu and Ni simultaneously can not only improve the corrosion resistance but also refine the structure and precipitation phase of the steel, significantly improving the low-temperature toughness. However, since both Cu and Ni are relatively expensive alloying elements, in order to minimize the cost of alloy design, the amount of Ni added to the steel of the present invention is 0.5% or less, preferably 0.3% or less.

[0032] Cr is an optional element in the present invention. Cr is added to steel mainly to improve the strength of the steel through solid solution strengthening or microstructure refinement. Since the structure of the steel of the present invention is fine bainitic ferrite + nano-precipitated carbides, 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. By adding a small amount of Cr, the yield strength of the steel can be appropriately reduced, thereby reducing the yield ratio. In addition, by adding a small amount of Cr, the corrosion resistance can also be improved. Usually, the addition amount of Cr is 0.5% or less, preferably 0.3% or less.

[0033] B is an optional element in the present invention. B can significantly improve the hardenability of steel, promote the bainite transformation, and accelerate the lath bainite transformation during the medium-temperature bainite phase transformation. Therefore, adding a small amount of B to steel is beneficial for obtaining a fine lath bainite structure. However, the B content should not be too high. Excessive addition of B will form martensite and more M-A islands, which is disadvantageous for plasticity and hole expansion. Therefore, the addition amount of B in the steel of the present invention is controlled to be 0.002% or less, preferably 0.001% or less.

[0034] O is an impurity element in the present invention. To obtain steel with better performance, the lower the O content in the steel, the better. However, a low oxidation amount will increase the steelmaking cost. To ensure the performance of strip steel, the O content in the steel of the present invention is controlled within 0.003%, preferably within 0.002%.

[0035] Existing high hole-expansion steel is usually designed with a high titanium composition. The main purpose of adding the microalloying element Ti is to refine the crystal grains, and the addition amount is generally within 0.1%. In the present invention, a high vanadium composition design is adopted, and Ti exists as an optional element in the steel of the present invention. The main purpose of adding V in the present invention is to combine V with C to form dispersed nano vanadium carbides for precipitation strengthening.

[0036] By setting the V content of the steel of the present invention to a maximum of 0.1 - 0.5%, steel having both high tensile strength and hole-expansion rate can be obtained. When the V content in the steel is 0.10 - 0.20%, the tensile strength of the steel is 590 MPa, and the hole-expansion rate is 70% or more, preferably 80% or more; when the V content in the steel is 0.20 - 0.35%, the tensile strength of the steel is 780 MPa, and the hole-expansion rate is 50% or more; when the V content in the steel is 0.35 - 0.50%, the tensile strength of the steel is 980 MPa, and the hole-expansion rate is 30% or more, preferably 40% or more.

[0037] Most of the microstructure of existing high-hole-expanded steel is ferrite or ferrite + bainite. In order to obtain higher strength, nano titanium carbide is used for strengthening. When the high-hole-expanded steel of the present invention does not contain Ti, its microstructure is bainite and nano vanadium carbide in bainite. Also, depending on different V contents, high-hole-expanded steels with different strength grades can be obtained, and the needs of downstream users for high-hole-expanded steels with different strength grades can be met. When the steel of the present invention contains Ti, its microstructure is ferrite and bainite, the ferrite contains nano TiC, and the bainite contains nano VC.

[0038] The present invention also provides a method for manufacturing the steel of the present invention, comprising the following steps: 1) A smelting and casting step, smelting the above composition in a converter or an electric furnace, performing secondary refining in a vacuum furnace, and casting it into a billet or an ingot; 2) A step of reheating the billet or the ingot, wherein the heating temperature is 1100 °C or higher, preferably 1200 °C or higher, and the holding time is 1 to 2 hours; 3) A hot rolling and cooling step and provides a method including the above steps.

[0039] As an embodiment of the manufacturing method of the present invention, preferably, in step 3) of the manufacturing method of the present invention, the initial rolling temperature is 1000 - 1100 °C, rough rolling is performed under high pressure at 950 °C or higher for 3 - 5 passes so that the cumulative deformation amount is 50% or more, then the intermediate billet is air-cooled or water-cooled to 900 - 950 °C, finish rolling is performed for 7 passes so that the cumulative deformation amount is 70% or more, finish rolling is completed between 800 - 900 °C to obtain a steel strip, and then the steel strip is water-cooled to 400 - 550 °C at a cooling rate of 10 °C / second or more and coiled, and slowly cooled to room temperature at a cooling rate of 20 °C / second or less to obtain a hot-rolled steel strip.

[0040] As another embodiment of the manufacturing method of the present invention, preferably, in step 3) of the manufacturing method of the present invention, the initial rolling temperature of hot rolling is 1050 - 1150°C, rough rolling is carried out under high pressure for 3 - 5 passes at 1050°C or higher so that the cumulative deformation amount becomes 50% or more. Next, the intermediate billet is heated to 950 - 1000°C, finish rolling is carried out for 3 - 7 passes so that the cumulative deformation amount becomes 70% or more, and the finish rolling temperature is set to 800 - 950°C to obtain a steel strip. The cooling is step cooling. After finish rolling, the steel strip is water-cooled to 600 - 750°C at a cooling rate of 30°C / second or more, then air-cooled for 1 - 10 seconds, and then the steel strip is cooled to 400 - 550°C at a cooling rate of 10°C / second or more and coiled. Next, it is cooled to room temperature at a cooling rate of 20°C / hour or less to obtain hot-rolled strip steel.

[0041] Preferably, when the steel of the present invention contains Ti, it is carried out using the above method including step cooling.

[0042] In step 3) above, the initial rolling temperature of hot rolling is 1050 - 1150°C, and rough rolling is carried out under high pressure for 3 - 5 passes at 1050°C or higher so that the cumulative deformation amount becomes 50% or more. The main purpose is to refine austenite crystal grains and at the same time retain more solid-solution Ti.

[0043] In the rough rolling and finish rolling stages of step 3), rolling should be completed as quickly as possible so that more Ti and V solidify in austenite. After hot finish rolling, the steel strip is first cooled to 600 - 750°C at a cooling rate of 30°C / second or more, and then nano-TiC in ferrite and ferrite crystal grains is formed in the air-cooling stage. Then, the steel strip is water-cooled to 400 - 550°C at a cooling rate of 10°C / second or more to obtain bainite and nano-precipitated VC. Finally, a fine structure mainly composed of ferrite and bainite, and nano-precipitated TiC and VC in ferrite and bainite is obtained.

[0044] When the steel contains both high titanium and high vanadium, the main purpose of adding more V is to combine V with C to form nano VC that is dispersed and precipitated to strengthen the precipitation. When the high titanium and high vanadium composition is combined with the step cooling process, nano TiC is formed in the ferrite grains in the ferrite formation region, and nano VC is formed in the bainite in the bainite formation region. Due to the combination of composition and process, the excessive Ti after binding with N can form nano TiC together with C in the air cooling stage after the first water cooling stage, which helps to strengthen the ferrite. Introducing nano TiC into the ferrite can reduce the performance difference between the ferrite and bainite, which is beneficial to improving the hole expansion rate. By controlling the strength of bainite with different V contents, high hole expansion steels with different strength grades can be obtained, which can meet the needs of downstream users for high hole expansion steels with different strength grades.

[0045] Preferably, the above method further includes 4) a pickling process. The pickling operation speed of the hot rolled strip steel is 30 - 120 m / min, the pickling temperature is 75 - 85 °C, the straightening rate is 3% or less, rinsing is carried out at 35 - 50 °C, and surface drying and oiling are carried out at 120 - 140 °C to obtain pickled high hole expansion steel.

[0046] In the method for manufacturing the steel of the present invention, The present invention adopts an intermediate temperature coiling process, which is combined with an innovative low carbon, high vanadium and low silicon composition design to obtain high strength high hole expansion steel with excellent performance stability.

[0047] In the intermediate temperature and low temperature bainite transformation regions, by accurately controlling the coiling temperature, uniform performance is ensured over the entire length of the strip steel. During the intermediate temperature coiling process, bainite phase transformation occurs with the nano precipitation of V. By quantifying the contents of the main elements C and V and combining them with specific coiling, a series of high hole expansion steel products with different strength grades and different hole expansion rates can be obtained.

[0048] Conventional high-titanium and high-hole-expanding steel mostly adopts a high-temperature coiling process, while the present invention adopts a medium-temperature coiling process. Therefore, in the rough rolling and finish rolling stages, the rolling pace should be completed as quickly as possible so that more V can be dissolved in austenite. After hot finish rolling, the strip steel is cooled online at a cooling rate of 10 °C / second or more to the medium-temperature range of 400 - 550 °C to obtain bainite and nano-precipitation structures.

[0049] When the steel contains Ti and step cooling is used in the manufacturing process, the desired amount of ferrite is formed in the air-cooling stage after the first water cooling after rolling, and nano-titanium carbides are precipitated in the ferrite crystal grains, improving the ferrite properties. In the medium-temperature coiling stage after the second water cooling, the precipitation of V is adopted to form nano-vanadium carbides in bainite, improving the strength of bainite. Through the innovation of composition and process, a ferrite strengthened by nano-titanium carbides and a bainite structure strengthened by nano-vanadium carbides can be obtained. Different series of high-hole-expanding steel products with different strength grades can be obtained through different V content designs, demonstrating the innovation of composition and process, as well as the uniqueness of the resulting structure and performance. Nano-TiC and VC respectively endow ferrite and bainite with higher strength and more balanced properties.

[0050] In the subsequent pickling process, the non-uniformity of thermal stress and tissue stress formed in the steel coil during the medium-temperature coiling phase transformation process can be completely removed during pickling and leveling, further improving the tissue uniformity. Based on this innovative design of composition and process, the present invention can obtain a series of high-surface hot-rolled pickled high-hole-expanding steel products with different strength grades, excellent plasticity, hole-expanding properties, and performance stability.

[0051] Compared with the prior art, the advantages or beneficial effects of the present invention are as follows.

[0052] Compared with the high-silicon composition design used in Chinese Patents CN103602895A and CN105821301A, the present invention adopts a silicon-free and high-vanadium composition design to avoid the appearance of red scale on the surface of strip steel and improve the surface quality of pickled high-strength steel.

[0053] When compared with Chinese Patent CN108570604A, its Si content is 0.05 - 0.5%, but it still cannot guarantee complete elimination of the red scale defects on the strip steel surface. Also, the three-stage cooling process is difficult to control and it is difficult to ensure performance stability. In contrast, the present invention adopts multi-pass cooling precision control technology and can ensure the performance uniformity of strip steel.

[0054] In the present invention, by further combining an innovative low-carbon, high-titanium, and high-vanadium composition design with innovative step cooling and medium-temperature coiling processes, hot-rolled pickled high-hole-expansion steels of different strength grades with excellent surface, strength, plasticity, and hole-expansion properties are obtained, and the performance stability is also good.

[0055] The steel of the present invention has a tensile strength of 590 MPa or more to 980 MPa or more, a thickness of 1.5 - 6.0 mm, good elongation (transverse direction A50 is 13% or more to 18% or more), and high hole-expansion property (hole-expansion rate is 30% or more to 80% or more), showing an excellent harmony of surface, strength, plasticity, and hole-expansion property, and can be applied to the manufacture of automotive chassis, subframes, and other complex parts that require high strength, thinning, hole-expansion, and flanging, with a very broad application prospect.

Brief Description of the Drawings

[0056]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0057] The present invention will be further described below with reference to examples and the accompanying drawings.

[0058] The steel compositions of Examples 1 to 18 of the present invention are shown in Table 1, and the balance is Fe and unavoidable impurities.

[0059] The process routes of the steel in Examples 1 to 18 of the present invention are as follows. 1) Smelting and casting process The composition shown in Table 1 was smelted in a converter or an electric furnace, subjected to secondary refining in a vacuum furnace, and cast into a billet or an ingot. 2) Process of reheating the billet or ingot In Examples 1 to 9, the heating temperature was 1100°C or higher and between 1100 and 1300°C, and in Examples 10 to 18, the heating temperature was 1200°C or higher and between 1200 and 1300°C. The holding time was 1 to 2 hours. 3) Hot rolling and cooling process In Examples 1 to 9, primary cooling was used. Specifically, the initial rolling temperature was 1000 to 1100°C, rough rolling of 3 to 5 passes was performed under high pressure at 950°C or higher so that the cumulative deformation amount became 50% or more, and then the intermediate billet was air-cooled or water-cooled to 900 to 950°C, finish rolling of 7 passes was performed so that the cumulative deformation amount became 70% or more, finish rolling was completed between 800 and 900°C, and then the steel strip was water-cooled to 400 to 550°C at a cooling rate of 10°C / second or more and wound up, and slowly cooled to room temperature at a cooling rate of 20°C / second or less. 4) Pickling process The pickling operation speed of the strip steel was 30 to 140 m / min, preferably 30 to 120 m / min, the pickling temperature was 75 to 85°C, the straightening rate was 3% or less, rinsing was performed at 35 to 50°C, and surface drying and oil coating were performed at 120 to 140°C.

[0060] In Examples 10 to 18, secondary cooling was used in the above step 3. Specifically, the initial rolling temperature of hot rolling was 1050 to 1150 °C, rough rolling of 3 to 5 passes was performed under high pressure at 1050 °C or higher so that the cumulative deformation amount became 50% or more. Next, the intermediate billet was heated to 950 to 1000 °C, finish rolling of 3 to 7 passes was performed so that the cumulative deformation amount became 70% or more, and the finish rolling temperature was 800 to 950 °C. The cooling was step cooling. After finish rolling, the steel strip was water-cooled to 600 to 750 °C at a cooling rate of 30 °C / second or more, air-cooled for 1 to 10 seconds, then the steel strip was cooled to 400 to 550 °C at a cooling rate of 10 °C / second or more and coiled, and cooled to room temperature at a cooling rate of 20 °C / hour or less.

[0061] Figure 1 shows the procedure of the manufacturing method of the present invention. Table 2 shows the steel manufacturing process parameters of Examples 1 to 18 of the present invention. Table 3 shows the performance parameters of the pickled steel of Examples 1 to 18 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] As can be seen from Table 1, none of the compositions of Comparative Examples 1 to 4 contain V element, and the compositions of Comparative Examples 1 to 3 contain high silicon. Therefore, from the viewpoint of the surface quality of the steel plate, the surfaces of the steel plates of Comparative Examples 1 to 3 inevitably contain red scale, but the examples of the present invention are all designed with a silicon-free composition and have excellent surface quality.

[0064] In addition, Comparative Examples 1 to 4 all adopt only a high titanium composition design and do not add V to the steel, which indicates that nano titanium carbide precipitation strengthening is adopted. On the other hand, the examples of the present invention adopt a high vanadium composition design and perform strengthening using nano vanadium carbide.

[0065] As can be seen from Table 3, through quantitative design and precise control of composition and main process parameters, the present invention can obtain high hole-expansion steels of three different strength grades with a yield strength of 500 MPa or more to 800 MPa or more, a tensile strength of 590 MPa or more to 980 MPa or more, an elongation (transverse direction A50) of 13% or more to 18% or more, and a hole expansion rate of 30% or more to 80% or more.

[0066] The mechanical properties of the steels in Table 3 were determined as follows.

[0067] The yield strength, tensile strength, and elongation (transverse direction A50) of the steels were tested in accordance with GB / T228.1-2021 "Tensile Test of Metallic Materials Part 1: Room Temperature Test Methods".

[0068] The hole expansion rate of the steels was tested in accordance with GB / T24524-2021 "Metallic Materials, Thin plates and stripes, Hole expansion test methods".

[0069] The steel microstructures of Examples 1 to 9 are bainite, the bainite contains nano VC, and the volume fraction of nano VC in the bainite is about 0.005 to 0.05%, preferably 0.005 to 0.03%.

[0070] The steel microstructures of Examples 10 to 18 are ferrite and bainite. The ferrite contains nano TiC, and the bainite contains nano VC. Specifically, the volume fraction of ferrite in the steel is 10 to 40%, the volume fraction of nano TiC in the ferrite is about 0.005 to 0.02%, the volume fraction of bainite in the steel is 60 to 90%, and the volume fraction of nano VC in the bainite is about 0.005 to 0.03%.

[0071] As can be seen from the above embodiments, the high-hole-expanded steel of the present invention exhibits a good harmony of high strength, high plasticity, and high hole expansion ratio, and is particularly suitable for manufacturing parts such as automobile chassis structures that require high strength, thin wall thickness, hole expansion, and flange forming, and has broad application prospects.

[0072]

Table 1

[0073]

Table 2

[0074]

Table 3

Claims

1. Steel containing the following components by mass percentage: C: 0.01 - 0.10%; Si ≤ 0.2%; Mn: 0.5 - 2.0%; P ≤ 0.02%; S ≤ 0.003%; Al: 0.01 - 0.08%; N ≤ 0.004%; V: 0.10 - 0.50%; O ≤ 0.003%; and the balance being Fe and unavoidable impurities.

2. The steel according to claim 1, further comprising Ti in a mass percentage of 0.05 - 0.2%, preferably 0.08 - 0.15%, more preferably 0.08 - 0.10%.

3. The steel according to claim 1 or 2, further comprising one or more components selected from Nb ≤ 0.1%, Cu ≤ 0.5%, Ni ≤ 0.5%, Cr ≤ 0.5%, and B ≤ 0.002% by mass percentage.

4. The steel according to claim 3, further comprising Mo in a mass percentage of 0.1 - 0.5%, preferably 0.20 - 0.40%, more preferably 0.2 - 0.3%.

5. The steel according to claim 1 or 2, wherein the components of the steel satisfy one or more of the following: C: 0.03 - 0.07%; Si ≤ 0.10%; Mn: 0.8 - 1.6%; S ≤ 0.0018%; Al: 0.02 - 0.05%; N ≤ 0.003%; O ≤ 0.002%.

6. The steel according to claim 3, wherein the components of the steel satisfy one or more of the following: Nb ≤ 0.06%, Cu ≤ 0.3%, Ni ≤ 0.3%, Cr ≤ 0.3%, and B ≤ 0.001%.

7. The steel according to claim 1, having a bainite and nano-precipitation VC in bainite structure.

8. The steel according to claim 2, having a ferrite and bainite structure, wherein the ferrite contains nano-TiC and the bainite contains nano-VC.

9. The steel according to any one of claims 1 - 8, having a yield strength of 500 MPa or more, a tensile strength of 590 MPa or more, preferably 780 MPa or more, and a transverse elongation A50 of 14% or more and 30% or less.

10. A method for manufacturing the steel according to any one of claims 1 - 9, comprising the following steps: 1) A smelting and casting step, smelting the composition according to any one of claims 1 - 9 in a converter or an electric furnace, secondary refining in a vacuum furnace, and casting into a billet or an ingot; 2) A step of reheating the billet or the ingot, a step with a heating temperature of 1100 °C or higher and a holding time of 1 to 2 hours, 3) a hot rolling and cooling step, and a method comprising the same.

11. In step 3), the initial rolling temperature is 1000 to 1100 °C, rough rolling is performed in 3 to 5 passes under high pressure at 950 °C or higher so that the cumulative deformation amount becomes 50% or more. Next, the intermediate billet is air-cooled or water-cooled to 900 to 950 °C, finish rolling is performed in 7 passes so that the cumulative deformation amount becomes 70% or more, and the finish rolling is completed between 800 and 900 °C to obtain a steel strip. After that, the steel strip is water-cooled to 400 to 550 °C at a cooling rate of 10 °C / second or more and coiled. Next, the steel strip is slowly cooled to room temperature at a cooling rate of 20 °C / second or less to obtain a hot-rolled steel strip. The method according to claim 10, characterized in that.

12. In step 3), the initial rolling temperature of the hot rolling is 1050 to 1150 °C, rough rolling is performed in 3 to 5 passes under high pressure at 1050 °C or higher so that the cumulative deformation amount becomes 50% or more. Next, the intermediate billet is heated to 950 to 1000 °C, finish rolling is performed in 3 to 7 passes so that the cumulative deformation amount becomes 70% or more, and the finish rolling temperature is set to 800 to 950 °C to obtain a steel strip. The cooling is step cooling. After the finish rolling, the steel strip is water-cooled to 600 to 750 °C at a cooling rate of 30 °C / second or more, then air-cooled for 1 to 10 seconds, and then the steel strip is cooled to 400 to 550 °C at a cooling rate of 10 °C / second or more and coiled. Next, the steel strip is cooled to room temperature at a cooling rate of 20 °C / hour or less to obtain a hot-rolled strip steel. The method according to claim 10, characterized in that.

13. 4) further comprising a pickling step, wherein the pickling operation speed of the hot-rolled strip steel is 30 to 140 m / min, preferably 30 to 120 m / min, the pickling temperature is 75 to 85 °C, the straightening rate is 3% or less, rinsing is performed at 35 to 50 °C, and surface drying and oil coating are performed at 120 to 140 °C. The method according to claim 11 or 12, characterized in that.

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