780MPa grade high surface high-performance stability ultra-high hole expansion steel and its manufacturing method

Abstract

A 780 MPa-class, high-surface, high-performance, stable, ultra-high hole-expanding steel and a method for producing the same, comprising a composition by weight percentage of 0.03-0.08% C, ≦0.2% Si, 0.5-2.0% Mn, ≦0.02% P, ≦0.003% S, 0.01-0.08% Al, ≦0.004% N, 0.05-0.20% Ti, 0.1-0.5% Mo, ≦0.005% Mg, ≦0.0030% O, with the balance being Fe and other unavoidable impurities. The ultra-high hole-expanding steel of the present invention has good microstructural uniformity, performance uniformity, and excellent matching of strength, plasticity, and ultra-high hole-expanding ratio, and has a yield strength of 750 MPa or more, a tensile strength of 780 MPa or more, and an elongation A 50 The hole expansion ratio is 15% or more and the hole expansion ratio is 70% or more; it also prevents the appearance of red iron scale on the steel plate surface and improves the surface quality of pickled high-strength steel; it fully meets user needs and can be used in passenger car chassis parts that require high strength and thin wall thickness, such as control arms and subframes.

Description

Technical Field

[0001] The present invention belongs to the field of high-strength steel, and particularly relates to a 780 MPa grade high-surface high-performance stability ultra-high hole-expanding steel and a manufacturing method thereof.

Background Art

[0002] Automobiles occupy a very important position in the development of the national economy. Many parts of passenger cars, especially some parts of the chassis and body, often need to use hot-rolled pickled products. The lightweighting of passenger cars is not only a development trend in the automotive industry but also a requirement of regulations. Although fuel consumption is regulated by regulations, in fact, it is a requirement for the lightweighting of the deformed body, which is reflected in the requirements for high-strength, thin-wall, and lightweight materials. High strength and lightweighting are inevitable requirements for subsequent new models, which will inevitably lead to an improvement in the grade of steel and a change in the chassis structure. For example, the parts become more complex, leading to improvements in material performance, surface requirements, and forming technologies such as hydroforming, hot stamping, and laser welding, and ultimately converting into performances such as high strength, stamping, flanging, springback, and fatigue characteristics of the material.

[0003] Compared with foreign countries, the development of high-strength high-hole-expanding steel in China not only has a relatively low strength level but also has poor performance stability. For example, the high-hole-expanding steel used by domestic automobile part manufacturers is basically high-strength steel with a tensile strength of 600 MPa or less, and the competition of high-hole-expanding steel at a level of 440 MPa or less is fierce. High-hole-expanding steel of the 780 MPa grade is currently gradually being used in batches in China, but the requirements for two important indicators in the forming process, namely elongation and hole-expanding rate, are also increasing.

[0004] While the domestic sales volume of passenger cars is gradually decreasing, an inflection point has emerged in the automotive industry, and competition is intensifying. In order to further reduce the cost of the process, passenger car manufacturers are seeking further performance improvements in materials. For example, in the manufacturing of structural parts of the control arm of an automobile chassis, in order to reduce the press working process, it is required to have high strength and high plasticity while further increasing the hole expansion rate index. The hole expansion rate of the 780MPa high hole expansion steel should be based on more than 50% of the current level and further increased to more than 70%. However, the current design concept of the existing 780MPa high hole expansion steel not only lacks the ability to ensure the performance of the production process, but also the hole expansion rate is basically 50 - 65%, and it cannot meet the higher hole expansion rate performance requirements submitted by users. In addition, the hole expansion rate is a parameter with large measurement variations, which is not only related to the uniformity and internal quality of the material structure, but also greatly related to the measurement process such as the quality of hole drilling and the determination of cracks in hole expansion.

[0005] As follows, there are many existing patents regarding 780MPa pickled high hole expansion steel. Chinese Patent Application CN103602895A relates to low-carbon Nb-Ti microalloyed high hole expansion steel. Its composition design features low carbon, high silicon, and Nb-Ti microalloying. The guaranteed value of the hole expansion rate is 50% or more. The design of the high silicon composition usually results in hematite scale on the surface of the steel plate. Also, the coiling temperature range required to form bainite is about 500°C, making it difficult to control the temperature of the entire length of the steel coil and likely to cause significant fluctuations in the performance of the entire length.

[0006] Chinese Patent Application CN105821301A relates to 800MPa hot-rolled high-strength high hole expansion steel. Its composition design features low carbon, high silicon, and Nb-Ti microalloying. Its Ti content reaches a very high level of 0.15 - 0.18%. In the actual manufacturing process, such a composition design concept not only has defects such as hematite scale on the surface of the steel strip, but also due to the ultra-high Ti content, it is easy to form coarse TiN, which is very disadvantageous to the stability of the hole expansion rate.

[0007] Chinese Patent Application CN108570604A relates to a hot-rolled pickled high hole-expanding steel of 780 MPa grade. Its composition design features low carbon, high aluminum, and high chromium, and a three-stage cooling process is adopted in the process design. There is no red iron scale on the surface of the strip, but due to the high aluminum design, clogging of the casting nozzle is likely to occur in the actual manufacturing process, the process is complex, the control of the three-stage cooling process is difficult, and the hole-expanding rate is not high.

Summary of the Invention

Problems to be Solved by the Invention

[0008] All of the above patent applications have problems such as red iron scale, difficulty in steelmaking, and difficulty in controlling the temperature uniformity of the entire length of the steel strip.

[0009] In order to meet the user's needs for higher surface quality, better performance stability, better strength, and better matching of plasticity and hole-expanding property, it is necessary to make innovative changes to the existing 780 MPa grade high hole-expanding steel.

[0010] As is well known, usually, the elongation rate of a material is inversely proportional to the hole-expanding rate, that is, the higher the elongation rate, the lower the hole-expanding rate, and conversely, the lower the elongation rate, the higher the hole-expanding rate. Therefore, it is very difficult to obtain a high hole-expanding steel with high elongation, high hole-expanding, and high strength.

[0011] Furthermore, under 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 hole expansion and flanging characteristics, a better balance between the two is required. On the other hand, in high-strength, high-plasticity, and high-hole-expansion steel, in order to improve the matching of strength, plasticity, and hole expansion, it seems indispensable to add more silicon elements. However, due to the high-silicon composition design, the surface of the steel becomes rough. That is, it is difficult to completely remove the defects of the hematite scale formed in the hot rolling process in the subsequent pickling process, and striped hematite scale appears on the surface of the pickled high-strength steel, which has a serious impact on the surface quality.

Means for Solving the Problems

[0012] An object of the present invention is to provide a high-surface, high-performance, stable, ultra-high-hole-expansion steel of 780 MPa grade and a manufacturing method thereof. The ultra-high-hole-expansion steel obtained by the method of the present invention has good tissue uniformity, performance uniformity, and excellent matching of strength, plasticity, and ultra-high hole expansion rate. Its yield strength is 750 MPa or more, tensile strength is 780 MPa or more, elongation A 50 is 15% or more, and the hole expansion rate is 70% or more; also, by adopting the method of the present invention, it is possible to avoid the appearance of hematite scale on the steel plate surface and improve the surface quality of pickled high-strength steel. The ultra-high-hole-expansion steel of the present invention can fully meet the needs of users and can be applied to chassis parts of passenger cars that require high strength and thinning, such as control arms and subframes.

[0013] To achieve the above object, the technical solution of the present invention is as follows. The high-surface, high-performance, stable, ultra-high-hole-expansion steel of 780 MPa grade has a weight percentage of composition of C: 0.03% - 0.08%, Si: ≤0.2%, Mn: 0.5% - 2.0%, P: ≤0.02%, S: ≤0.003%, Al: 0.01 - 0.08%, N: ≤0.004%, Ti: 0.05% to 0.20%, Mo: 0.1% to 0.5%, Mg: ≤ 0.005%, O: ≤ 0.0030%, and the balance is Fe and other inevitable impurities.

[0014] The extra-high hole-expanding steel of the present invention may further contain one or more of Nb, V, Cu, Ni, Cr, B, and Ca. Preferably, when contained, Nb ≤ 0.06%, V ≤ 0.05%, Cu ≤ 0.5%, Ni ≤ 0.5%, Cr ≤ 0.5%, B ≤ 0.001%, Ca ≤ 0.005%; more preferably, the contents of the above-mentioned Cu, Ni, and Cr are each preferably 0.3% or less, the contents of the above-mentioned Nb and V are each preferably 0.03% or less, the content of the above-mentioned B is preferably 0.0005% or less, and the content of the above-mentioned Ca is preferably 0.002% or less.

[0015] In some embodiments, the extra-high hole-expanding steel of the present invention contains at least one or any two of Nb, V, Cu, Ni, Cr, B, and Ca.

[0016] In some embodiments, the thickness of the extra-high hole-expanding steel of the present invention is 1.5 to 6 mm, preferably 2 to 6 mm.

[0017] The yield strength of the extra-high hole-expanding steel described in the present invention is 750 MPa or more, preferably 760 MPa or more, the tensile strength is 780 MPa or more, preferably 810 MPa or more, and the elongation A 50 is 15% or more, and the hole-expanding rate is 70% or more, preferably 80% or more, more preferably 90% or more.

[0018] The microstructure of the extra-high hole-expanding steel described in the present invention is bainite + nano-scale carbide, and the nano-scale carbide is precipitated in bainite and ferrite.

[0019] In the composition design of the extra-high hole-expanding steel described in the present invention, Carbon is a basic element in steel and is also one of the important elements for the present invention. Carbon expands the austenite phase region and stabilizes austenite. As an interstitial atom in steel, carbon plays a very important role in improving the strength of steel and has the greatest influence on the yield strength and tensile strength of steel. In the present invention, since the structure obtained in the hot rolling stage is low-carbon bainite, in order to obtain high-strength steel with a final tensile strength of 780 MPa grade, it is necessary to ensure that the carbon content is 0.03% or more. When the carbon content is 0.03% or less, the tensile strength of bainite formed during low-temperature coiling is low, and it is difficult to achieve a strength of 780 MPa or more after bell-type annealing. However, the carbon content does not exceed 0.08. If the carbon content is too high, it is easy to form low-carbon martensite during low-temperature coiling. Therefore, the carbon content needs to be controlled within 0.03 - 0.08%, and the preferred range is 0.04 - 0.07%.

[0020] Silicon is a basic element in steel. As described above, in order to meet the requirements of high strength, high plasticity, and high hole expansion rate submitted by users, relatively more silicon is usually added to the composition design. However, a high-silicon composition design leads to a deterioration in the surface quality of the steel sheet and an increase in the defects of red iron scale. In the present invention, in order to ensure good surface quality, it is necessary to strictly control the silicon content during the composition design. That is, although silicon is an impurity element in the present invention, considering that silicon manganese is used for deoxidation in actual steelmaking, it is difficult to completely avoid adding silicon. According to a large number of statistical data in actual production, when the silicon content is 0.2% or less, defects of surface red iron scale can be avoided during hot rolling, and it can be ensured that there is no red iron scale, usually 0.15% or less. Also, when the silicon content is 0.2% or more, it is easy to form a martensite structure during online cooling or low-temperature coiling, and it is difficult to obtain a bainite structure. Therefore, the silicon content in steel needs to be controlled within 0.2%, and the preferred range is within 0.15%. In some embodiments, the Si content is 0.05 - 0.2%. In some other embodiments, the Si content is 0.05 - 0.15%.

[0021] Manganese is the most basic element in steel and is also one of the important elements for the present invention. It is well known that Mn is an important element that reduces the critical hardening rate of steel, stabilizes austenite, refines crystal grains, and expands the austenite phase region that delays the transformation of 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 should not exceed 2.0% usually. Otherwise, segregation of Mn is likely to occur during steelmaking, and hot cracks are also likely to occur during continuous casting of slabs. Therefore, the Mn content in steel is usually controlled to be 0.5 - 2.0%, and the preferred range is 1.0 - 1.6%.

[0022] Phosphorus is an impurity element in steel. P is likely to segregate at grain boundaries. When the P content in steel is high (≥0.1%), Fe2P is formed and precipitates around the crystal grains, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better. Generally, it is controlled within 0.02% preferably without increasing the steelmaking cost.

[0023] Sulfur is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. Especially when the contents of S and Mn are relatively high, a large amount of MnS is formed in the steel. MnS itself has a certain degree of plasticity, and in 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 disadvantageous for hole expansion performance. Therefore, the lower the S content in steel, the better. However, in order to reduce the MnS content, it is necessary to strictly control the S content, and the S content needs to be controlled below 0.003%, and the preferred range is below 0.0015%.

[0024] Aluminum mainly serves as a deoxidizer and a nitrogen fixer in steel. When there are strong carbide-forming elements such as Ti present, the main roles of Al are deoxidation and grain refinement. In the present invention, Al is a common deoxidizing element and an element for refining the crystal grains, and its content is usually controlled to be 0.01 - 0.08%. If the Al content is less than 0.01%, there will be no effect on refining the crystal grains. Similarly, if the Al content exceeds 0.08%, the grain refinement effect will saturate. Therefore, the Al content in the steel should be controlled to be 0.01 - 0.08%, and the preferred range is 0.02 - 0.05%.

[0025] Nitrogen is an impurity element in the present invention, and the lower its content, the better. However, nitrogen is an inevitable element in the steelmaking process. Although its content is small, when it combines with strong carbide-forming elements such as Ti, the formed TiN particles have a very adverse effect on the properties of the steel, especially the hole expansion performance. Since TiN is square, there is a large stress concentration between its sharp corners and the matrix. During the process of hole expansion deformation, the stress concentration between TiN and the matrix is likely to form cracks, significantly reducing the hole expansion performance of the material. Since the present invention adopts a high-titanium design for the composition system, in order to minimize the adverse effect of TiN on hole expansion, the nitrogen content needs to be controlled below 0.004%, and the preferred range is below 0.003%.

[0026] Titanium is one of the important elements in the present invention. Ti mainly plays two roles in the present invention. One is to combine with the impurity element N in the steel to form TiN and play the role of "nitrogen fixation"; the other is to form uniform and fine nano-carbides in bainite and ferrite during the subsequent high-temperature bell-type annealing process to improve strength, plasticity and hole expandability. When the titanium content is less than 0.05%, no obvious precipitation strengthening effect can be obtained. When the titanium content exceeds 0.20%, the impact toughness of the steel plate is likely to decrease due to coarse TiN. Therefore, the titanium content in the steel needs to be controlled to be 0.05 - 0.20%, and the preferred range is 0.07 - 0.10%.

[0027] Molybdenum is one of the important elements in the present invention. When molybdenum is added to steel, the phase transformation between ferrite and pearlite can be significantly delayed, which is advantageous for obtaining a bainite structure. Also, molybdenum has strong resistance to softening due to welding. The main objective of the present invention is to obtain a low-carbon bainite structure. Since low-carbon bainite tends to soften after welding, adding a certain amount of molybdenum can effectively reduce the degree of welding softening. Therefore, the molybdenum content needs to be controlled within 0.1 - 0.5%, and the preferred range is 0.20 - 0.40%.

[0028] Magnesium is one of the important elements in the present invention. When magnesium is added to steel, fine MgO dispersed in the steelmaking stage can be preferentially formed. These fine MgO can be used as nucleation particles for TiN, effectively increasing the nucleation points 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, it is likely to make the hole expansion rate unstable. Therefore, the Mg content in the steel can be controlled within 0.005%.

[0029] Oxygen is an element that inevitably exists in the steelmaking process. In the present invention, generally, after deoxidation, the oxygen content in the steel becomes 30 ppm or less, which will not have a significant adverse effect on the performance of the steel plate. Therefore, the oxygen content in the steel can be controlled within 30 ppm.

[0030] Copper is an element that can be added in the present invention. When copper is added to steel, the corrosion resistance of the steel can be improved. When added together with P element, the corrosion resistance effect is better. When the addition amount of Cu exceeds 1%, under specific conditions, an ε-Cu precipitation phase is formed, exerting a strong precipitation strengthening effect. However, since the addition of Cu is likely to cause the "Cu embrittlement" phenomenon in the rolling process, in order to fully exert the corrosion resistance improvement effect of Cu in a certain application without causing the "Cu embrittlement" phenomenon too much, usually the content of Cu element is controlled within 0.5%, and the preferred range is within 0.3%.

[0031] Nickel is an element that can be added in the present invention. When nickel is added to steel, it has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper. Adding nickel to steel has little effect on the tensile properties of steel, but it can refine the structure and precipitation phase of steel, and can greatly improve the low-temperature toughness of steel. At the same time, in steel added with copper, adding a small amount of nickel can suppress the occurrence of "Cu embrittlement". Even if a relatively large amount of nickel is added, there is no obvious adverse effect on the properties of the steel itself. When copper and nickel are added simultaneously, not only the corrosion resistance is improved, but also the structure and precipitation phase of steel are refined, and the low-temperature toughness is greatly improved. However, since both copper and nickel are relatively expensive alloying elements, in order to minimize the cost of alloy design, the addition amount of nickel is usually 0.5% or less, and the preferred range is 0.3% or less.

[0032] Chromium is an element that can be added in the present invention. Adding chromium to steel mainly improves the strength of steel by means such as solid-solution strengthening or refinement of the structure. Since the structure of the present invention is fine bainite, ferrite and nano-precipitated carbides, after undergoing a high-temperature bell-type annealing process, the mobile dislocations in the structure decrease, and the ratio of the yield strength to the tensile strength of the steel, that is, the yield ratio, becomes relatively high, usually reaching 0.90 or more. Adding a small amount of chromium element can appropriately reduce the yield strength of steel, thereby reducing the yield ratio. Furthermore, adding a small amount of chromium also plays a role in improving the corrosion resistance. Usually, the addition amount of chromium is 0.5% or less, and the preferred range is 0.3% or less.

[0033] Niobium is an element that can be added in the present invention. Similar to titanium, niobium is a strong carbide element in steel. When niobium is added to steel, the unrecrystallized temperature of the steel significantly increases, and deformed austenite with a high dislocation density can be obtained in the finish rolling stage, and the final phase transformation structure can be refined in the subsequent transformation process. However, the addition amount of niobium should not be too much. On the other hand, when the addition amount of niobium exceeds 0.06%, relatively coarse niobium carbonitrides are likely to be formed in the structure, consuming some carbon atoms and reducing the precipitation strengthening effect of carbides. At the same time, when the niobium content is high, anisotropy of the austenite structure in the hot rolling form easily occurs and is inherited by the final structure during the subsequent cooling phase transformation process, which is disadvantageous for the hole expansion performance. Therefore, the niobium content in steel is usually controlled below 0.06%, and the preferred range is below 0.03%.

[0034] Vanadium is an element that can be added in the present invention. Similar to titanium and niobium, vanadium is also a strong carbide-forming element. However, the solution temperature or precipitation temperature of vanadium carbide is low, and it usually dissolves completely into austenite in the finish rolling stage. Vanadium only begins to form in ferrite when the temperature drops and the phase transformation starts. Since the solubility of vanadium carbide in ferrite is larger than that of niobium and titanium, the size of vanadium carbide formed in ferrite becomes larger, which is disadvantageous for precipitation strengthening. Although it contributes to the strength of steel, it is smaller than titanium carbide. However, the formation of vanadium carbide also consumes a certain amount of carbon atoms, which is disadvantageous for improving the strength of steel. Therefore, the addition amount of vanadium in steel is usually below 0.05%, and the preferred range is below 0.03%.

[0035] Boron is an element that can be added in the present invention. Boron significantly improves the hardenability of steel and is advantageous for obtaining a martensite structure. Considering that the expected structure in the hot rolling stage of the present invention is bainite rather than martensite, it is necessary to strictly control the content of boron element in the steel in order to prevent the formation of martensite due to excessive addition of boron element. Furthermore, when boron element is added to steel, it may form irregular ferrite structures or even martensite-austenite components that are disadvantageous to the low-temperature impact toughness of the steel. Therefore, the added content of boron in the steel is usually controlled to be 0.001% or less, and the preferred range is 0.0005% or less.

[0036] Calcium is an element that can be added in the present invention. Calcium can improve the morphology of sulfides such as MnS, and change long strip-shaped sulfides such as MnS into spherical CaS, which is advantageous for improving the morphology of inclusions, thereby reducing the adverse effect of long strip-shaped sulfides on hole expansion performance. However, if too much calcium is added, the amount of calcium oxide increases, which is disadvantageous for hole expansion performance. Therefore, the added amount of calcium in the steel is usually 0.005% or less, and the preferred range is 0.002% or less.

[0037] The ultra-high hole expansion steel described in the present invention adopts the design concept of a silicon-free or low-silicon composition, and precipitates nano-scale carbides uniformly dispersed in bainite and ferrite. Uniform and fine bainite, and uniformly distributed nano-scale carbides formed in bainite and ferrite give the steel plate excellent tissue uniformity, performance uniformity, and excellent matching of strength, plasticity, and ultra-high hole expansion rate.

[0038] The manufacturing method of the 780 MPa grade high surface high performance stability ultra-high hole expansion steel described in the present invention includes the following steps: 1) Smelting and casting According to the above composition, smelt by adopting a converter or an electric furnace, and after secondary refining in a vacuum furnace, cast into a billet or an ingot; 2) Reheat the billet or ingot, with a heating rate of 20 °C / h or more, a heating temperature of 1230 °C or more, and a holding time of 1 to 2 hours; 3) Hot rolling Set the rolling start temperature at 1050 - 1150 °C, perform rough rolling in 3 - 5 passes at 1050 °C or more so that the cumulative deformation amount is 50% or more. Then, hold the intermediate billet at 950 - 1000 °C, and perform finish rolling in 3 - 7 passes so that the cumulative deformation amount is 70% or more; set the finish rolling temperature at 850 - 950 °C; cool the steel plate to 300 °C or below and wind it up; 4) Annealing Adopt bell - type annealing, with a heating rate of 20 °C / h or more, a bell - type annealing temperature of 500 - 650 °C, and a bell - type annealing time of 12 - 48 h; cool the steel plate to 300 °C or below at a cooling rate of 50 °C / h or less and take it out of the furnace, and wind it up; Return ; 5) Pickling Adjust the pickling running speed of the steel strip within the range of 30 - 140 m / min, control the pickling temperature between 75 - 85 °C, Elongation rate in the tension leveling process control it to 3% or less, then perform rinsing, dry the surface of the steel strip, and apply oil.

[0039] Preferably, after pickling, perform rinsing in the temperature range of 35 - 50 °C to ensure the surface quality of the steel strip, and dry the surface of the steel strip between 120 - 140 °C and apply oil.

[0040] In the manufacturing method described in the present invention: The heating temperature of the billet (ingot) is 1230 °C or more, the holding time is 1 - 2 hours, the rolling start temperature is 1050 - 1150 °C, and the main purpose of performing rough rolling in 3 - 5 passes at 1050 °C or more so that the cumulative deformation amount is 50% or more is to refine austenite crystal grains while retaining more solid - solution titanium; then, hold the intermediate billet at 950 - 1000 °C, perform rolling in 3 - 7 passes so that the cumulative deformation amount is 70% or more, after the finish rolling between 850 - 950 °C is completed, water - cool the steel plate to 300 °C or below at a cooling rate of 50 °C / s or less, and cool it to room temperature after winding.

[0041] In some embodiments, the heating rate in step 2) is 20 to 40 °C / h, and the heating temperature is 1230 to 1300 °C.

[0042] In the rough rolling and finish rolling stages, in order to ensure more dissolved titanium in austenite, it is necessary to complete the rolling rhythm as quickly as possible. After hot finish rolling, the steel strip is cooled online at a cooling rate of 10 to 100 °C / second, preferably 30 to 100 °C / second, to 300 °C or lower to obtain a low-carbon bainite structure.

[0043] By annealing the steel coil obtained in the hot rolling process at a high temperature of 500 to 650 °C for a long time, the precipitation of nanoscale carbides in bainite and ferrite is promoted, and at the same time, the structure and precipitation are made more uniform. In the hot rolling stage, low-carbon bainite with a uniform and fine structure is formed by low-temperature coiling.

[0044] In the bell-type annealing stage, fine and dispersed nanoscale carbides precipitate in bainite and ferrite, improving strength, plasticity, and hole expansion rate. There is an inverse relationship between the bell-type annealing temperature and time. The lower the bell-type annealing temperature, the longer the bell-type annealing time; conversely, the higher the bell-type annealing temperature, the shorter the bell-type annealing time. When the bell-type annealing temperature is less than 500 °C, the carbides do not precipitate sufficiently, and when the bell-type annealing temperature exceeds 650 °C, the carbides tend to coarsen and the strength decreases. Therefore, the bell-type annealing temperature is selected between 500 and 650 °C.

[0045] In some embodiments, the heating rate in step 4) is 20 to 40 °C / h, and the cooling rate is 15 to 50 °C / h.

[0046] In the steelmaking process, the Mg deoxidation method is used to preferentially form dispersed fine MgO in the molten steel, generate more nucleation particles for forming TiN in the subsequent continuous casting process, effectively refine the TiN particles, and improve the hole expansion rate stability.

[0047] When a relatively large amount of Ti and a small amount of Mo are added, nano-scale carbides (Ti, Mo)C can be further precipitated from bainite under the conditions of a long-time bell-type annealing process, so that the tensile strength can reach 780 MPa or more. The addition of the Mo element can ensure that the precipitated nano-scale carbides do not significantly coarsen at a higher bell-type annealing temperature and do not affect the strength of the final steel strip.

[0048] After the steel strip undergoes the above low-temperature coiling and high-temperature bell-type annealing processes, it exhibits excellent strength, plasticity, and hole expansion properties, and particularly has excellent performance stability.

[0049] In the bell-type annealing process, the uniform and fine low-carbon bainite obtained by low-temperature coiling in the hot rolling stage can promote the precipitation of nano-scale carbides in bainite and ferrite by the high-temperature bell-type annealing process, and at the same time, the uniformity of the structure can be further improved.

[0050] After passing through the pickling process, it also has excellent surface quality. The above composition, process, structure, and performance not only have completely different design concepts compared with the existing 780 MPa high hole expansion steel, but also have stable performance, simple process, and are suitable for mass production.

[0051] The present invention has the following beneficial effects. 1. The ultra-high hole expansion steel described in the present invention adopts the design concept of no silicon or low silicon in the composition design. Silicon is an impurity element in the present invention. The design of no silicon or low silicon can ensure that molten fayalite (Fe2SiO4) is not formed on the surface in the slab heating process. The advantages of the design of no silicon or low silicon are as follows: First, it is beneficial to obtain excellent steel strip surface quality; second, it is beneficial to accurately control the main process parameters in the hot rolling process to obtain a hot rolled steel strip with stable performance over the whole length; third, it is beneficial to obtain a uniform bainite structure in the subsequent low-temperature coiling process.

[0052] 2. In the process design, a low-temperature winding + bell-type annealing process is adopted, which is quite different from the conventional high-temperature winding high hole-expansion steel. By combining a silicon-free or low-silicon composition design with a low-temperature winding process, a hot-rolled steel strip with uniform and stable properties throughout its entire length can be obtained, and the structure is full bainite. Since the nano-precipitation phase during low-temperature winding is too slow to completely precipitate, the strength of the bainite structure at this time is usually less than 780 MPa. Furthermore, through the subsequent bell-type annealing process, fine and uniform nano-carbides are further formed in bainite and ferrite, resulting in a further improvement in strength to 780 MPa or more. The temperature of the steel coil under the bell-type annealing process conditions is relatively uniform, and the nano-precipitation in bainite and ferrite is also more uniform than the nano-precipitation during the phase transformation process directly during high-temperature winding, ensuring that the steel strip has uniform and stable properties throughout its entire length. Uniform and fine bainite, and uniformly distributed nano-scale carbides formed in bainite and ferrite give the steel plate excellent structural uniformity, performance uniformity, and an excellent matching of strength, plasticity, and ultra-high hole-expansion rate.

[0053] 3. The steel coil or steel plate of the high-surface high-performance stability ultra-high hole-expansion steel described in the present invention has a yield strength of 750 MPa or more, a tensile strength of 800 MPa or more, a thickness of 1.5 - 6 mm, preferably 2 mm - 6 mm, and at the same time has good elongation (transverse direction A 50 ≧15%), cold bending performance (d ≦ 2.5a, 180°), and hole-expansion performance (hole-expansion rate ≧ 70%). It shows an excellent matching of strength, plasticity, toughness, cold bending, and hole-expansion performance, and can be applied to the manufacture of complex parts that require high strength, thin thickness, and hole-expansion / flanging processing, such as the chassis and subframes of automobiles, and has a very broad application prospect.

Brief Description of the Drawings

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Figure 6

Embodiments for Carrying Out the Invention

[0055] Hereinafter, the present invention will be further described based on examples and drawings. Referring to FIGS. 1 to 3, the manufacturing method of the 780 MPa grade high surface high performance stability ultra-high hole expansion steel described in the present invention includes the following steps: 1) Smelting and casting According to the above composition, smelting was carried out by adopting a converter or an electric furnace, and after secondary refining in a vacuum furnace, it was cast into a billet or an ingot; 2) As shown in FIG. 2, the billet or ingot was reheated, with a heating rate of 20 °C / h or more, a heating temperature of 1230 °C or more, and a holding time of 1 to 2 hours; 3) Hot rolling As shown in FIG. 3, the rolling start temperature was set to 1050 - 1150 °C, rough rolling was carried out in 3 - 5 passes at 1050 °C or more so that the cumulative strain was 50% or more, then the intermediate billet was held at 950 - 1000 °C, and finish rolling was carried out in 3 - 7 passes so that the cumulative strain was 70% or more; the finish rolling temperature was set to 850 - 950 °C; 4) Annealing As shown in Fig. 4, bell annealing was adopted, with a heating rate of 20 °C / h or more, a bell annealing temperature of 500 - 650 °C, and a bell annealing time of 12 - 48 h; the steel plate was cooled to 300 °C or below at a cooling rate of 50 °C / h or less and taken out of the furnace, and coiled Return was carried out; 5) Pickling The pickling running speed of the steel strip was adjusted within the range of 30 - 140 m / min, the pickling temperature was controlled between 75 - 85 °C, Elongation rate in the tension leveling process was controlled to 3% or less, rinsing was carried out in the temperature range of 35 - 50 °C, the surface of the steel strip was dried between 120 - 140 °C, and oil was applied.

[0056] The composition of the examples of the ultra-high hole-expanding steel described in the present invention is referred to Table 1, Tables 2 and 3 are the manufacturing process parameters of the examples of the steel of the present invention, among which, the thickness of the steel billet in the rolling process is 120 mm; Table 4 is the mechanical properties of the steel plates of the examples of the present invention. The tensile properties (yield strength, tensile strength, elongation) were tested according to the ISO6892-2-2018 international standard, and the hole-expanding rate was tested according to the ISO16630-2017 international standard.

[0057] From Table 4, it can be seen that the yield strength of the steel coil is 750 MPa or more, the tensile strength is 800 MPa or more, the elongation A 50 is usually 16 - 18%, and the hole-expanding rate is 70% or more. From the above examples, the 780 MPa high-strength steel according to the present invention has a good matching of strength, plasticity, toughness and hole-expanding property, and is particularly suitable for high-strength and thin-walled parts such as the chassis structure of automobiles, and parts that require hole-expanding and flange forming (such as control arms, etc.), and is also suitable for complex parts that require hole flange forming such as wheels, and it can be seen that there is a very broad application prospect.

[0058] Figures 4, 5, and 6 respectively provide representative metallographic photographs in the case of direct online cooling and low-temperature winding in Examples 1, 3, and 7 of the present invention. From the figures, it can be seen that in the composition system designed according to the present invention, the structure obtained during low-temperature winding is uniform and fine low-carbon bainite. In the subsequent bell-type annealing process, nano-carbides finely and uniformly dispersed in bainite and ferrite laths precipitate, thereby improving strength and plasticity, improving the uniformity of the structure, and improving hole expansion performance.

[0059]

Table 1

[0060]

Table 2

[0061]

Table 3

[0062]

Table 4

Claims

1. The weight percentages of the composition are as follows: C: 0.03 - 0.08%, Si: ≤0.2%, Mn: 0.5 - 2.0%, P: ≤0.02%, S: ≤0.003%, Al: 0.01 - 0.08%, N: ≤0.004%, Ti: 0.05 - 0.20%, Mo: 0.1 - 0.5%, Mg: ≤0.005%, O: ≤0.0030%, and the balance being Fe and other inevitable impurities, a hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion, wherein the microstructure of the hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion is bainite + nano-scale carbide, and the nano-scale carbide is precipitated in the ferrite of the bainite. The above-mentioned 780 MPa grade high-surface high-performance stable ultra-high hole-expanding hot-rolled steel sheet has a yield strength of 750 MPa or more, a tensile strength of 780 MPa or more, and an elongation A ５０ of 15% or more and a hole-expanding rate of 70% or more, which is a 780 MPa grade high-surface high-performance stable ultra-high hole-expanding hot-rolled steel sheet.

2. The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 1, further containing any one or any two or more of Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5%, Cr≤0.5%, B≤0.001%, and Ca≤0.005%.

3. Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5%, Cr≤0.5%, B≤0.001%, and Ca≤0.005%, and the composition satisfies the content of Cu being 0.3% or less; the content of Ni being 0.3% or less; the content of Cr being 0.3% or less; the content of Nb being 0.03% or less; the content of V being 0.03% or less; the content of B being 0.0005% or less; and the content of Ca being 0.002% or less; The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 2, satisfying one or more of the above characteristics.

4. The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 1, wherein the C content is 0.04 - 0.07%.

5. The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 1, wherein the Si content is 0.15% or less, the S content is 0.0015% or less, and / or the N content is 0.003% or less.

6. The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 1, wherein the Mn content is 1.0 - 1.6%.

7. The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 1, wherein the Al content is controlled to be 0.02 - 0.05%.

8. The hot-rolled steel sheet of 780 MPa grade with high surface high performance and high stability for ultra-high hole expansion according to Claim 1, wherein the Ti content is 0.07 - 0.10%.

9. The 780 MPa grade hot-rolled steel sheet with high surface, high performance, and stable ultra-high hole expansion performance according to Claim 1, wherein the Mo content is 0.20 to 0.40%.

10. The above-mentioned extra-high hole-expanding hot-rolled steel sheet has a yield strength of 760 MPa or more, a tensile strength of 810 MPa or more, an elongation A ５０ of 15% or more, and a hole-expanding rate of 80% or more. The 780 MPa grade high surface high-performance stability extra-high hole-expanding hot-rolled steel sheet according to claim 1.

11. A method for manufacturing the 780 MPa grade hot-rolled steel sheet with high surface, high performance, and stable ultra-high hole expansion performance according to Claims 1 to 10, comprising the following steps: 1) Smelting and casting Smelt according to the composition described in any one of Claims 1 to 9 by using a converter or an electric furnace, perform secondary refining in a vacuum furnace, and then cast into a billet or an ingot; 2) Reheat the billet or ingot, with a heating rate of 20 °C / h or more, a heating temperature of 1230 °C or more, and a holding time of 1 to 2 hours; 3) Hot rolling Set the rolling start temperature to 1050 to 1150 °C, perform rough rolling in 3 to 5 passes at 1050 °C or more so that the cumulative reduction is 50% or more. Then, hold the intermediate billet at 950 to 1000 °C, and further perform finish rolling in 3 to 7 passes so that the cumulative reduction is 70% or more; set the finish rolling temperature to 850 to 950 °C; cool the steel sheet to 300 °C or less and then wind it up; 4) Annealing Adopt bell-type annealing, with a heating rate of 20 °C / h or more, a bell-type annealing temperature of 500 to 650 °C, and a bell-type annealing time of 12 to 48 h; cool the steel sheet to 300 °C or less at a cooling rate of 50 °C / h or less, take it out of the furnace, and unwind it; 5) Pickling Adjust the pickling running speed of the steel strip within the range of 30 to 140 m / min, control the pickling temperature between 75 and 85 °C, control the elongation rate in the tension leveling process to 3% or less, then perform rinsing, dry the surface of the steel strip, and apply oil.

12. The method for manufacturing the 780 MPa grade hot-rolled steel sheet with high surface, high performance, and stable ultra-high hole expansion performance according to Claim 11, wherein in step 5), rinsing is performed in the temperature range of 35 to 50 °C, and the surface of the steel strip is dried between 120 and 140 °C and then oil is applied.

13. The method for manufacturing the 780 MPa grade hot-rolled steel sheet with high surface, high performance, and stable ultra-high hole expansion performance according to Claim 11, wherein in step 2), the heating rate is 20 to 40 °C / h and the heating temperature is 1230 to 1300 °C.

14. The method for manufacturing the 780 MPa grade hot-rolled steel sheet with high surface, high performance, and stable ultra-high hole expansion performance according to Claim 11, wherein in step 4), the heating rate is 20 to 40 °C / h and the cooling rate is 15 to 50 °C / h.

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