A 500mpa grade precipitation-strengthened high-strength steel and a manufacturing method

Abstract

The application discloses a 500MPa grade precipitation strengthening type high-strength steel and a manufacturing method. Chemical component composition and mass percentage thereof are as follows: C: 0.035-0.055%, Si: 0.05-0.09%, Mn: 1.00-1.20%, P: 0.010% or less, S: 0.006% or less, Al: 0.08-0.11%, V: 0.13-0.17%, Bi: 0.03-0.05%, and the balance of Fe and inevitable impurities. The manufacturing method comprises the following steps: steelmaking, hot rolling, coiling, pickling, continuous annealing / hot-dip galvanizing. The microstructure of the high-strength steel is ferrite and pearlite, and a large number of dispersed precipitates are uniformly distributed in the ferrite matrix and grain boundary; the strength fluctuation of the head, middle and tail of the coil is less than or equal to 20MPa; the yield point elongation can be eliminated without large flattening; the high-strength steel is easy to be stably produced in industrialization, and has excellent comprehensive performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel production and manufacturing, and particularly relates to a 500MPa grade precipitation strengthening type high-strength steel and a manufacturing method thereof. BACKGROUND

[0002] Low-alloy high-strength steel is widely used in manufacturing automobile structural parts and reinforcing parts, and the steel has the characteristics of high yield ratio, good plasticity, excellent weldability, etc. The 500MPa grade low-alloy high-strength steel is mostly micro-alloyed with niobium and titanium, and the strength of the steel can be significantly improved by using the fine-grain strengthening effect of niobium and titanium elements. Titanium ferroalloy is relatively low in price compared with niobium ferroalloy, but if argon blowing station or ladle refining is used in the steelmaking process, the titanium alloy is severely burned, so the steel grade with added titanium element in each steel plant is refined through a vacuum station. The vacuum treatment is 50-80 yuan / ton of steel higher in cost than argon blowing station or ladle refining, which increases the smelting cost. Due to the fine-grain strengthening mechanism of niobium and titanium and the rolling capacity limitation of the flattening machine, it is difficult to eliminate the yield platform of the steel coil, resulting in a high yield point elongation of the steel coil in the annealing or hot-dip galvanizing flattening supply state. If the yield point elongation is too high, the surface of the stamped part is prone to "orange peel" phenomenon, which leads to part scrap, which increases the difficulty of industrial production.

[0003] Patent publication No. CN106893933A (Shougang General Company) discloses a "production method of low-alloy high-strength steel", which uses single Nb micro-alloying. In order to reduce the yield point elongation value, 0.15-0.35% Si content is added. As for the addition of Si, it is known to those skilled in the art that such a high Si content is not conducive to hot-dip galvanizing of low-alloy high-strength steel. Si can form Mn2SiO4 compounds with Mn and O in the steel and accumulate on the surface of the steel plate, which can easily lead to steel plate plating leakage. At the same time, the yield strength of the material of the invention is only 419-444 MPa, which cannot meet the demand of higher strength grade.

[0004] Patent application publication CN104399781A (Wuhan Iron and Steel Group) discloses a method for eliminating the yield platform of low-alloy high-strength steel. In order to eliminate the yield platform of low-alloy high-strength steel, the skin pass mill uses a small roller diameter work roll of 420-435 mm to improve the skin pass elongation. This method can only solve the problem of low-alloy high-strength steel with a yield strength of less than 420 MPa. For higher strength grades of low-alloy high-strength steel, using this method will result in excessive rolling force of the skin pass mill (≥10000 KN), causing the risk of shutdown. Furthermore, because industrial production is continuous, there are many varieties of automobile sheets produced, and other steel grades that do not require large skin pass amounts use large diameter work rolls. Temporarily changing to small diameter work rolls according to this patent method will affect the production rhythm and lead to a decrease in production efficiency, increasing the production cost per unit of time. The steel grade has mass production and manufacturability only under the premise of stable cost and performance. Therefore, the chemical composition system of the steel sheet needs to be changed to solve the problems of cost and performance. Vanadium micro-alloying precipitation strengthening can achieve the strengthening of low-alloy high-strength steel, and can achieve the same strengthening effect as niobium-titanium micro-alloying. The yield point extension phenomenon of the finished product can be easily eliminated, and the 0.050-0.10% residual vanadium in the vanadium ore alloy molten iron raw material can be fully utilized, greatly reducing the smelting raw material cost of the steel coil, and solving the above problems. SUMMARY

[0005] The present application provides a 500MPa grade precipitation strengthening type high-strength steel and a manufacturing method. The method uses vanadium precipitation strengthening based on carbon manganese steel, and through unique steelmaking, hot rolling and cold rolling process design, achieves the purpose of low-cost manufacturing while improving the comprehensive performance of the product, solving the defects and problems described in the background art.

[0006] To achieve the above object, the technical scheme adopted by the present application is as follows: a 500MPa grade precipitation strengthening type high-strength steel, the chemical composition and mass percentage of the high-strength steel are as follows: C: 0.035-0.055%, Si: 0.05-0.09%, Mn: 1.00-1.20%, P≤0.010%, S≤0.006%, Al: 0.08-0.11%, V: 0.13-0.17%, Bi: 0.03-0.05%, and the balance is Fe and unavoidable impurities.

[0007] The chemical composition of the high-strength steel according to the present application further contains the following elements and their mass percentages: Cr≤0.15%, Mo≤0.09%, Cu≤0.15%, Ni≤0.30%, N≤0.03%, and the balance is Fe and unavoidable impurities.

[0008] The high-strength steel has yield strength of 500-525 MPa, tensile strength of 560-640 MPa, elongation of 20-28%, strain hardening index n of 0.15-0.21, and strength difference of ≤20 MPa at the beginning, middle and end of coiling.

[0009] The high-strength steel has yield point elongation of ≤0.3%, forming limit FLC0 of ≥0.25, and hole expansion ratio of ≥134%, and the surface quality can reach O5 level.

[0010] The application also provides a manufacturing method of the 500 MPa grade precipitation strengthening type high-strength steel, which comprises the steps of steelmaking, hot rolling, coiling, pickling, continuous annealing / hot dip galvanizing, the steelmaking process comprises the steps of molten iron desulfurization, converter smelting, LF furnace refining and continuous casting, and the specific control parameters are as follows:

[0011] (1) the steelmaking process: the molten iron desulfurization is to S≤0.005%, the converter smelting is followed by LF furnace refining for micro-alloying and nitrogen blowing, and then the continuous casting is performed to obtain a casting blank;

[0012] (2) the hot rolling and coiling process: the casting blank obtained in the step (1) is directly hot-rolled, the final rolling temperature is controlled to be 855-885 ℃, the coiling temperature is 560-580 ℃, and the hot-rolled coil thickness is 4.5-5.5 mm;

[0013] (3) the pickling and continuous annealing process: the hot-rolled coil material in the step (2) is rolled to 1.3-2.1 mm through pickling, and then is annealed through a continuous annealing production line, the flattening elongation is 0.3-0.7%, and then is coiled to obtain a cold-rolled product;

[0014] (4) the pickling and hot dip galvanizing process: the hot-rolled coil material in the step (2) is rolled to 1.3-2.1 mm through pickling, and then is heated to the soaking temperature of 790-810 ℃ through a hot dip galvanizing production line, and then is cooled to the final cooling temperature of 420-440 ℃, and then the steel plate is heated to the temperature of 460-465 ℃ for entering the zinc pot, and then is subjected to hot dip galvanizing treatment in the zinc pot, the zinc liquid temperature is controlled to be 455-465 ℃, the flattening elongation is controlled to be 0.3-0.7%, and then is air-cooled to 140-160 ℃, and then is coiled to obtain a hot dip galvanized product.

[0015] In the step (1) of the steelmaking process, the molten iron contains vanadium in an amount of 0.050-0.10%.

[0016] In order to ensure that the steel plate has excellent comprehensive performance, the basic principles for limiting the content of each element are as follows:

[0017] C: is a solid solution strengthening element, if the content of C is too low, the strength of the steel is difficult to guarantee, and if the content of C is too high, the plasticity and welding performance of the steel will be reduced.

[0018] Si: can play a role in inhibiting cementite formation, Si content is too low, can not play the above role, Si content is too high, the surface quality of the material is poor, and it is difficult to galvanize.

[0019] Mn: is a conventional toughening element, too high Mn content, on the one hand, increases the cost, and also makes the welding structure appear hardening layer, leading to crack weld and heat affected zone crack sensitivity increases.

[0020] P: the lower the P in the material, the better, mainly to avoid the secondary processing brittleness of the material.

[0021] S: the lower the content of S in the steel, the better, to avoid the adverse effects of cracks and welding on the steel material of the present application. In addition, high sulfur content will consume too much manganese element, affecting the strengthening effect of the steel, or increasing the cost of the material.

[0022] Al: part of the aluminum in the steelmaking link plays a deoxidizing role, and part of the Al plays a precipitated strengthening role of Al (C, N), and too high Al content will seriously block the crystallizer nozzle in the steelmaking process in industrial production.

[0023] V: is a strong carbonitride forming element, which is beneficial to precipitation strengthening and can prevent high temperature austenite from growing too much, but too high content can easily lead to carbonitride segregation, poor machinability, and increased material cost. Make full use of the 0.050~0.10% residual vanadium in the enterprise's internal vanadium ore alloy molten iron raw material itself, which greatly reduces the smelting raw material cost of the steel coil.

[0024] Bi: in the present application, bismuth has the effect of increasing the stamping forming performance of the steel plate due to its volume cold expansion performance, in addition, bismuth also plays a role in enhancing the welding performance of the steel plate and increasing the strength of the steel plate.

[0025] Cr, Mo: are solid solution strengthening elements and strong carbide forming elements, but too high content leads to increased cost and also deteriorates the forming performance of the steel.

[0026] Cu, Ni: enhance the precipitation strengthening effect and corrosion resistance of the steel plate.

[0027] N: in the present application, nitrogen forms V (C, N) precipitates with vanadium and carbon, which significantly improves the precipitation strengthening effect of the steel.

[0028] The beneficial effects produced by the above technical solution are: 1. Through reasonable chemical composition design and process design, especially, the V micro-alloying precipitation strengthening of the steel plate realizes the strengthening of the low-alloy high-strength steel, and the finished product yield point elongation is ≤0.3% under the low light elongation of 0.3-0.7%, the strength fluctuation of the head, middle and tail of the coil is small (≤20 MPa); 2. The steel plate stamping forming performance is increased by fully utilizing the volume cold expansion performance of Bi element, the forming limit FLC0 is ≥0.25, the hole expansion rate is ≥134%, and Bi also plays a role in enhancing the welding performance of the steel plate and increasing the strength of the steel plate, so that the C and Mn addition amount of the material is reduced under the condition of 500 MPa level strength; 3. The high-strength steel has high surface quality and can reach O5 level, and can be used to manufacture car body cross beams and reinforcing parts; 4. The low flattening (lightening) can eliminate the yield point elongation, avoid the surface "orange peel" defects of the steel plate stamping automobile parts, reduce the raw material production cost, reduce the rolling force of the flattening (lightening) machine, reduce the equipment wear and tear, and improve the production efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Metallographic structure of example 1 (a) cold-rolled continuous annealing plate;

[0030] Figure 2 Metallographic structure of example 5 (b) hot-dip galvanized plate;

[0031] Figure 3 Scanning electron microscope graph of example 1;

[0032] Figure 4 Transmission electron microscope graph of example 1. DETAILED DESCRIPTION

[0033] The application will be further described in detail below in combination with the drawings and specific embodiments, but the embodiments should not be understood as limiting the application.

[0034] Examples 1-8

[0035] The manufacturing method of the 500 MPa grade precipitation strengthening type high-strength steel includes steelmaking, hot rolling, coiling, pickling, continuous annealing / hot-dip galvanizing processes, the steelmaking process includes molten iron desulfurization, converter smelting, LF furnace refining, continuous casting blank steps;

[0036] Among them, examples 1-4 are obtained by using steelmaking, hot rolling, coiling, pickling, continuous annealing processes to obtain cold-rolled steel plates; examples 5-8 are obtained by using steelmaking, hot rolling, coiling, pickling, hot-dip galvanizing processes to obtain hot-dip galvanized steel plates; the steelmaking, hot rolling and pickling processes of the cold-rolled steel plates and the hot-dip galvanized steel plates are the same, and the specific control method is as follows:

[0037] (1) Steelmaking process: desulphurization of molten iron to S≤0.005%, micro-alloying and nitrogen blowing after converter smelting by LF furnace refining, and then continuous casting into billets.

[0038] (2) Hot rolling and coiling process: the billets obtained in step (1) are directly hot rolled, the finish rolling temperature is controlled at 855-885°C, the coiling temperature is 560-580°C, and the hot rolled coil thickness is 4.5-5.5 mm; the control parameters of each example are shown in Table 1.

[0039] Table 1 Control parameters of steelmaking, hot rolling and coiling process of Examples 1-8

[0040]

[0041] (3) Pickling, continuous annealing process: after the hot rolled steel coil of step (2) is rolled to 1.3-2.1 mm by pickling, it is annealed by continuous annealing production line, the flat extension rate is 0.3-0.7%, and then coiled into a coil; the control parameters of each example are shown in Table 2.

[0042] Table 2 Control parameters of pickling, continuous annealing process of Examples 1-4

[0043]

[0044] (4) Pickling, hot dip galvanizing process: after the hot rolled steel coil of step (2) is rolled to 1.3-2.1 mm by pickling, it is annealed by hot dip galvanizing production line, the soaking temperature is 790-810°C, the slow cooling temperature is 690-710°C, the fast cooling end temperature is 420-440°C, then heated to the zinc pot plate temperature of 460-465°C, then hot dip galvanizing treatment is carried out in the zinc pot, the zinc liquid temperature is controlled at 455-465°C, the finishing extension rate is controlled at 0.3-0.7%, and then air cooled to 140-160°C, and coiled into a steel coil; the control parameters of each example are shown in Table 3.

[0045] Table 3 Control parameters of pickling, hot dip galvanizing process of Examples 5-8

[0046]

[0047] According to the above process, the specific chemical composition of the 500 MPa grade precipitation strengthening type high strength steel obtained in each example is shown in Table 4, and the properties of each example are shown in Table 5, wherein Examples 1-4 are cold rolled and continuous annealing treatment delivery products, and Examples 5-8 are hot dip galvanizing delivery products.

[0048] Table 4 Chemical composition of steel plate (wt%) of Examples 1-8

[0049]

[0050] Table 5 Mechanical properties of steel plate of Examples 1-8

[0051]

[0052] By Figure 1 , 2 It can be seen that the microstructure of the cold-rolled and hot-galvanized products observed at 500 times of metallographic magnification is consistent, and the grain size of the products is 14, and the microstructure is ferrite + pearlite, and the pearlite content is about 50%. By Figure 2 , 3 It can be seen that a large amount of vanadium carbonitride is dispersedly precipitated in the ferrite matrix and at the grain boundaries (the drawings of the remaining examples are similar, and are omitted).

[0053] The above examples are only used to illustrate but not to limit the technical solutions of the present application. Although the present application has been described in detail with reference to the above examples, it should be understood by those skilled in the art that the present application can still be modified or equivalently replaced without departing from the spirit and scope of the present application, and any modification or partial replacement should be covered in the scope of the claims of the present application.

Claims

1. A 500 MPa grade precipitation-strengthened high-strength steel, characterized in that, The high-strength steel chemical composition and its mass percentage are as follows: C: 0.035-0.055%, Si: 0.05-0.09%, Mn: 1.00-1.20%, P≤0.010%, S≤0.006%, Al: 0.08-0.11%, V: 0.13-0.17%, Bi: 0.03-0.05%, and the balance of Fe and inevitable impurities; The high-strength steel has a tensile strength of 560-640 MPa and a structure of ferrite+pearlite. The yield point elongation of the finished high-strength steel is ≤0.3%, the forming limit FLC0 is ≥0.25, the hole expansion ratio is ≥134%, and the surface quality can reach O5 level.

2. The 500 MPa grade precipitation-strengthening high-strength steel according to claim 1, characterized in that, The high-strength steel chemical composition further contains the following elements and their mass percentages: Cr≤0.15%, Mo≤0.09%, Cu≤0.15%, Ni≤0.30%, N≤0.03%, and the balance of Fe and inevitable impurities.

3. The 500 MPa grade precipitation-strengthening high-strength steel according to claim 1 or 2, characterized in that, The high-strength steel has a yield strength of 500-525 MPa, an elongation of 20-28%, a strain hardening index n of 0.15-0.21, and a strength difference of ≤20 MPa at the beginning, middle and end of the coil.

4. The method of claim 1-3, wherein the method of manufacturing a 500 MPa grade precipitation-strengthened high-strength steel is characterized by: The manufacturing method comprises the steps of steelmaking, hot rolling, coiling, pickling, continuous annealing / hot dip galvanizing, and the steelmaking process comprises the steps of molten iron desulfurization, converter smelting, LF furnace refining, and continuous casting; and the specific control parameters are as follows: (1) Steelmaking process: the molten iron desulfurization is to S≤0.005%, the converter smelting is followed by LF furnace refining for micro-alloying and nitrogen blowing, and then the continuous casting is performed to form a casting blank; (2) Hot rolling and coiling process: the casting blank obtained in step (1) is directly hot-rolled, the final rolling temperature is controlled at 855-885℃, the coiling temperature is 560-580℃, and the hot-rolled coil thickness is 4.5-5.5mm; (3) Pickling and continuous annealing process: the hot-rolled coil material of step (2) is rolled to 1.3-2.1mm by pickling, then annealed by continuous annealing production line, the flattening elongation is 0.3-0.7%, and then coiled to obtain a cold-rolled product; (4) Pickling and hot dip galvanizing process: the hot-rolled coil material of step (2) is rolled to 1.3-2.1mm by pickling, then heated to a soaking temperature of 790-810℃ by hot dip galvanizing production line, and then cooled to a final cooling temperature of 420-440℃, then the steel plate is heated to a temperature of 460-465℃ before entering the zinc pot, then the hot dip galvanizing treatment is performed in the zinc pot, the zinc liquid temperature is controlled at 455-465℃, the flattening elongation is controlled at 0.3-0.7%, and then air-cooled to 140-160℃, and finally coiled to obtain a hot dip galvanized product.

5. The method of producing a 500 MPa grade precipitation-strengthening high-strength steel according to claim 4, characterized in that, In the steelmaking process of step (1), the molten iron contains 0.050-0.10% vanadium.

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