Ultra-low carbon martensite and retained austenite type extra-high hole expansion steel of 980 MPa grade and production method thereof

Abstract

A 980 MPa-class ultra-low carbon martensite and retained austenite type ultra-high hole-expanding steel and a method for producing the same, wherein the weight percentages of the chemical composition are C: 0.03% to 0.06%, Si: 0.8% to 2.0%, Mn: 1.0% to 2.0%, P: ≦0.02%, S: ≦0.003%, Al: 0.02 to 0.08%, N: ≦0.004%, Mo: 0.1% to 0.5%, Ti: 0.01% to 0.05%, O: ≦0.0030%, and the balance is Fe and other unavoidable impurities. The ultra-high hole-expanding steel described in the present invention has a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, and an elongation (transverse A 50 ) is 10% or more, cold bending performance is (d≦4a, 180°), and the hole expansion ratio is 80% or more, making it suitable for use in chassis parts of passenger cars 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 980 MPa grade ultra-low carbon martensite and retained austenite type super high hole expansion steel and a manufacturing method thereof.

Background Art

[0002] With the development of the national economy, the production of automobiles has also increased significantly, and the use of sheet materials continues to increase. Many parts of the domestic automobile industry need to use plates that are hot-rolled or pickled according to the conventional design. For example, chassis parts of automobiles, torsion beams, subframes of vehicles, spokes and rims of wheels, front and rear axle assemblies, body structure parts, seats, clutches, seat belts, truck box panels, protection nets, spare parts such as automobile beams, etc. Among them, the steel for chassis accounts for 24 - 34% of the total steel consumption of automobiles.

[0003] 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 body with a changed shape, which is reflected in the requirement for high-strength, thin-walled, and lightweight materials. High strength and lightweighting are inevitable requirements for subsequent new models, which will inevitably lead to an improvement in the steel grade and a change in the chassis structure. For example, the parts will become more complex, leading to improvements in requirements such as material performance and surface, as well as forming technologies such as hydroforming, hot stamping, and laser welding, and ultimately, conversions in performances such as the high strength, stamping, flanging, springback, and fatigue characteristics of the materials.

[0004] Compared with foreign countries, the development of high-strength and high-hole-expanding steel in the country not only has a relatively low strength level but also inferior 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. The high-hole-expanding steel of the 780 MPa grade is now gradually beginning to be used in batches, but the requirements for two important indicators in forming, namely elongation and hole-expanding rate, are also increasing. The 980 MPa grade high-hole-expanding steel is still in the research and development and certification stage and has not reached the mass production stage. However, the 980 high-hole-expanding steel with higher strength and higher hole-expanding rate is an inevitable development trend in the future. To meet the potential needs of future users, it is necessary to develop 980 MPa grade high-hole-expanding steel with excellent hole-expanding properties.

[0005] Currently, most of the relevant patent documents are high-hole-expanding steels of 780 MPa and below. There are very few documents on 980 MPa grade high-hole-expanding steel. Chinese patent application CN106119702A discloses 980 MPa grade hot-rolled high-hole-expanding steel, and the main feature of its composition design is low-carbon V-Ti micro-alloy design. The microstructure is granular bainite and a small amount of martensite, and at the same time, a small amount of Nb and Cr are added.

Summary of the Invention

Problems to be Solved by the Invention

[0006] As can be seen from the literature, usually, the elongation of the material is inversely proportional to the hole-expanding rate, that is, the higher the elongation, the lower the hole-expanding rate, and conversely, the lower the elongation, the higher the hole-expanding rate. Therefore, it is very difficult to obtain high-hole-expanding steel with high elongation, high hole-expanding, and high strength.

[0007] 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 / flanging characteristics, a better balance between the two is required. Of course, the hole expansion rate of the material is closely related to many factors, and the most important factors include the uniformity of the structure, the level of inclusion and segregation control, various structure types, and the measurement of the hole expansion rate. Generally speaking, a single uniform structure is advantageous for obtaining a higher hole expansion rate, while a two-phase or multi-phase structure is usually disadvantageous for improving the hole expansion rate.

[0008] The object of the present invention is to provide a 980 MPa grade ultra-low carbon martensite and retained austenite type ultra-high hole expansion steel and its manufacturing method. The high hole expansion steel is a high-strength and high hole expansion steel with a yield strength of 800 MPa or more and a tensile strength of 980 MPa or more, and can be applied to chassis parts of passenger cars that require high strength and thinning, such as control arms and subframes.

Means for Solving the Problems

[0009] To achieve the above object, the technical solution of the present invention is as follows. The composition design of the present invention adopts a relatively low C content, thereby ensuring that the user has excellent weldability during use and ensuring that the obtained low-carbon martensite structure has good hole expansion properties and impact toughness. Based on satisfying the tensile strength ≧ 980 MPa, the lower the carbon content, the better; the Si content is designed to be high to obtain more retained austenite according to the process, thereby improving the plasticity of the material. At the same time, a higher Si content is advantageous for lowering the unrecrystallized temperature of the steel, enabling the steel to complete the dynamic recrystallization process at a lower finish rolling temperature, thereby refining the sizes of the austenite grain and the final martensite grain, and improving the plasticity and hole expansion rate.

[0010] Specifically, the 980 MPa grade ultra-low carbon martensite and retained austenite type super high hole expansion steel described in the present invention has a chemical composition by weight percentage of C: 0.03% to 0.06%, Si: 0.8% to 2.0%, Mn: 1.0% to 2.0%, P: ≤ 0.02%, S: ≤ 0.003%, Al: 0.02 to 0.08%, N: ≤ 0.004%, Mo: 0.1% to 0.5%, Ti: 0.01% to 0.05%, O: ≤ 0.0030%, and the balance is Fe and other inevitable impurities.

[0011] Furthermore, the 980 MPa grade ultra-low carbon martensite and retained austenite type super high hole expansion steel described in the present invention further contains one or more elements of Cr ≤ 0.5%, B ≤ 0.002%, Ca ≤ 0.005%, Nb ≤ 0.06%, V ≤ 0.05%, Cu ≤ 0.5%, and Ni ≤ 0.5%.

[0012] In some embodiments, the 980 MPa grade ultra-low carbon martensite and retained austenite type super high hole expansion steel described in the present invention has a chemical composition by weight percentage of C: 0.03% to 0.06%, Si: 0.8% to 2.0%, Mn: 1.0% to 2.0%, P: ≤ 0.02%, S: ≤ 0.003%, Al: 0.02 to 0.08%, N: ≤ 0.004%, Mo: 0.1% to 0.5%, Ti: 0.01% to 0.05%, O: ≤ 0.0030%, Cr: ≤ 0.5%, B: ≤ 0.002%, Ca: ≤ 0.005%, Nb: ≤ 0.06%, V: ≤ 0.05%, Cu: ≤ 0.5%, Ni: ≤ 0.5%, and the balance is Fe and other inevitable impurities; preferably, the 980 MPa grade ultra-low carbon martensite and retained austenite type super high hole expansion steel described in the present invention contains at least one of Cr, B, Ca, Nb, V, Cu, and Ni, preferably at least Cr and / or B.

[0013] The Cr content is preferably 0.2 to 0.4%; the Cu and Ni contents are each preferably 0.3% or less; the Nb and V contents are each preferably 0.03% or less; the B content is preferably 0.0005 to 0.0015%; and the Ca content is preferably 0.002% or less.

[0014] Furthermore, the microstructure of the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion steel described in the present invention is bainite and a small amount of retained austenite. Preferably, the proportion of retained austenite in the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion steel is 5% or less by volume ratio.

[0015] Furthermore, the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion steel described in the present invention has a yield strength of 800 MPa or more, preferably 815 MPa or more, a tensile strength of 980 MPa or more, preferably 1000 MPa or more, more preferably 1030 MPa or more, and the transverse direction A 50 is 10% or more, passes the cold bending performance test (d≤4a, 180°), and the hole expansion rate is 80% or more, preferably 85% or more, more preferably 90% or more. Preferably, the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion steel described in the present invention has a -40°C impact toughness of 140 J or more, preferably 150 J or more, more preferably 160 J or more.

[0016] Preferably, the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion steel described in the present invention has a yield strength of 815 MPa or more, a tensile strength of 1000 MPa or more, and the transverse direction A 50 is 10% or more, passes the cold bending performance test (d≤4a, 180°), the hole expansion rate is 85% or more, and the -40°C impact toughness is 150 J or more.

[0017] Preferably, the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion steel described in the present invention has a thickness of 2 to 6 mm.

[0018] In the composition design of the 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole expansion 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 to be obtained is low-carbon or ultra-low-carbon martensite, in order to obtain high-strength steel with a tensile strength of 980 MPa grade, it is necessary to ensure that the carbon content is 0.03% or more. Otherwise, if the carbon content is 0.03% or less, even if it is fully quenched to room temperature, its tensile strength cannot reach 980 MPa; however, the carbon content does not exceed 0.06%. If the carbon content is too high, the strength of the formed low-carbon martensite will be too high, and both the elongation and hole expansion rate will be low. Therefore, the carbon content needs to be controlled within 0.03 - 0.06%, and the preferred range is 0.04 - 0.055%.

[0019] Silicon is a basic element in steel and is also one of the important elements for the present invention. The increase in Si content not only improves the solid solution strengthening effect, but more importantly, plays the following two roles. One of them is to significantly lower the unrecrystallized temperature of the steel so that the steel can complete dynamic recrystallization in a very low temperature range. In this way, in the actual rolling process, rolling can be carried out at a relatively low finishing rolling temperature. For example, rolling can be carried out within the finishing rolling temperature range of 800 - 850 °C, which can significantly reduce the size of austenite grain size, thereby reducing the size of the final martensite lath, improving strength and plasticity, and also being advantageous for obtaining a good hole expansion rate. Another important role of Si is to suppress the precipitation of cementite. Under appropriate rolling process conditions, especially when a martensite-based structure is obtained, a certain amount of retained austenite can be retained, which is advantageous for improving the elongation rate. As is well known, under the condition of the same strength level, the elongation rate of martensite is usually the lowest. To improve the elongation rate of martensite, retaining a certain amount of stable retained austenite is an important means. Such a role of Si is only manifested when its content reaches 0.8% or more. However, the Si content should not be too high, otherwise, the rolling load in the actual rolling process will increase, which is disadvantageous for the stable production of products. Therefore, the Si content in steel is usually controlled at 0.8 - 2.0%, and the preferred range is 1.2 - 1.6%.

[0020] 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 stabilize retained austenite, the Mn content is usually controlled to be 1.0% or more. At the same time, the Mn content should not normally exceed 2.0%, otherwise, segregation of Mn is likely to occur during steelmaking, and hot cracks are also likely to occur during continuous slab casting. Therefore, the Mn content in steel is usually controlled to be 1.0 - 2.0%, and the preferred range is 1.4 - 1.8%.

[0021] Phosphorus is an impurity element in steel. P tends to segregate at grain boundaries. When the P content in steel is high (≧0.1%), Fe2P is formed and precipitates around crystal grains, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better, and it is usually controlled within 0.02% to avoid increasing the steelmaking cost.

[0022] 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 during the subsequent rolling process, MnS deforms along the rolling direction, not only reducing the plasticity in the transverse direction of the steel but also increasing the anisotropy of the structure, which is disadvantageous for hole expansion performance. Therefore, the lower the S content in steel, the better. However, considering that it is necessary to increase the Mn content in the present invention, in order to reduce the MnS content, it is necessary to strictly control the S content. The S content needs to be controlled within 0.003%, and the preferred range is 0.0015% or less.

[0023] Aluminum mainly serves as a deoxidizer and a nitrogen fixer in steel. In the presence of strong carbide-forming elements such as Ti, Nb, and V, 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 0.02 - 0.08%. If the Al content is less than 0.02%, there is no effect of refining the crystal grains. Similarly, if the Al content exceeds 0.08%, the grain refinement effect becomes saturated. Therefore, the Al content in the steel may be controlled to be 0.02 - 0.08%, and the preferred range is 0.02 - 0.05%.

[0024] 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 combined 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 hole expansion deformation process, the stress concentration between TiN and the matrix is likely to form cracks, significantly reducing the hole expansion performance of the material. On the premise of minimizing the nitrogen content as much as possible, the lower the content of strong carbide-forming elements such as Ti, the better. In the present invention, in order to minimize the adverse effect of TiN, a small amount of Ti is added to fix nitrogen. Therefore, the nitrogen content needs to be controlled to be 0.004% or less, and the preferred range is 0.003% or less.

[0025] 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 a certain amount of dispersed fine TiN in the subsequent welding process of the material, suppress the size of austenite crystal grains, refine the structure, and improve the low-temperature toughness. Therefore, the Ti content in the steel is controlled in the range of 0.01 - 0.05%, preferably in the range of 0.01 - 0.03%.

[0026] Molybdenum is one of the important elements in the present invention. When added to steel, molybdenum can significantly delay the phase transformation between ferrite and pearlite. Such an effect of molybdenum is beneficial for the adjustment of various processes in the actual rolling process. For example, after the finish rolling, divided cooling may be carried out, or water cooling may be carried out after air cooling. In the present invention, a process of water cooling after air cooling or directly water cooling after rolling is adopted. By adding molybdenum, it is possible to prevent the formation of structures such as ferrite or pearlite during the air cooling process. At the same time, during the air cooling process, the deformed austenite can dynamically recover, which is beneficial for improving the uniformity of the structure. Molybdenum has strong resistance to welding softening. The main purpose of the present invention is to obtain a structure of single low-carbon martensite and a small amount of retained austenite. Since low-carbon martensite 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 0.15 - 0.35%.

[0027] Chromium is one of the elements that can be added in the present invention. Adding a small amount of chromium element is not for improving the hardenability of steel, but is beneficial for forming acicular ferrite in the heat-affected zone after welding by combining with B, and significantly improving the low-temperature toughness of the heat-affected zone of the weld. Since the final application part related to the present invention is a passenger car chassis product, the low-temperature toughness of the heat-affected zone of the weld is a very important index. In addition to ensuring that the strength of the heat-affected zone of the weld does not decrease excessively, the low-temperature toughness of the heat-affected zone of the weld also needs to meet specific requirements. Furthermore, chromium itself also has a certain effect of preventing welding softening. Therefore, the addition amount of chromium in the steel is usually 0.5% or less, and the preferred range is 0.2 - 0.4%.

[0028] Boron is one of the elements that can be added in the present invention. The role of boron in steel is mainly to segregate at the original austenite grain boundaries to suppress the formation of proeutectoid ferrite; when boron is added to steel, the hardenability of the steel is also significantly improved. However, in the present invention, the main purpose of adding a trace amount of boron element is not to improve the hardenability, but to combine with chromium to improve the structure in the heat-affected zone of welding and obtain a needle-like ferrite structure with good toughness. The addition of boron element in steel is usually controlled below 0.002%, and the preferred range is 0.0005 - 0.0015%.

[0029] 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 beneficial to improving the morphology of inclusions, thereby reducing the adverse effect of long strip-shaped sulfides on the hole expansion property. However, if too much calcium is added, the amount of calcium oxide increases, which is disadvantageous to the hole expansion property. Therefore, the addition amount of calcium in steel is usually below 0.005%, and the preferred range is below 0.002%.

[0030] Oxygen is an element that inevitably exists in the steelmaking process. In the present invention, generally after deoxidation, the O content in steel becomes below 30 ppm, and it will not have a significant adverse effect on the performance of the steel plate. Therefore, the O content in steel can be controlled within 30 ppm.

[0031] Niobium is one of the elements 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 rises significantly, 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%.

[0032] 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 begins to form in ferrite only when the temperature drops and the phase transition 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 and contributes to the strength of steel, but is smaller than that of titanium. 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%.

[0033] Copper is an element that can be added in the present invention. When added to steel, copper can improve the corrosion resistance of steel. 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, which plays 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, the content of Cu element is usually controlled within 0.5%, and the preferred range is within 0.3%.

[0034] Nickel is an element that can be added in the present invention. When nickel is added to steel, it has a 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 significantly improve the low-temperature toughness of steel. At the same time, in the 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 significantly 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.

[0035] The manufacturing method of the 980MPa grade ultra-low carbon martensite and retained austenite type ultra-high hole expansion steel of 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 the heating temperature being 1100 - 1200°C and the holding time being 1 - 2 hours; 3) Hot rolling Set the rolling start temperature to 950 - 1100°C, and perform 3 - 5 at 950°C or above so that the cumulative deformation amount is 50% or more PathPerform rough rolling, then hold the intermediate billet at 900 - 950°C, preferably 920 - 950°C, and further perform finish rolling so that the cumulative strain is 70% or more, preferably 85% or more, with 3 - 5 Path passes; set the finish rolling temperature to 800 - 920°C; 4) Cooling First, air cool for 0 - 10 seconds, then water cool the steel strip to below the martensite transformation start point Ms at a cooling rate of 50°C / s or more, preferably 50 - 85°C / s, and wind it up. After winding up, cool to room temperature, and the cooling rate is preferably 20°C / h or less; 5) Pickling Adjust the pickling running speed of the steel strip within the range of 30 - 100 m / min, control the pickling temperature between 75 - 85°C, control the tension correction rate to 2% or less to reduce the loss of elongation of the steel strip, then perform rinsing, dry the surface of the steel strip, and apply oil.

[0036] Preferably, in step 5), after pickling, perform rinsing in the temperature range of 35 - 50°C, dry the surface of the steel strip at 120 - 140°C, and apply oil.

[0037] The innovation points of the present invention are as follows. In the composition design, the present invention adopts the design concept of low-carbon or ultra-low-carbon martensite, adds relatively high silicon to suppress and reduce the formation of cementite, and at the same time, adding relatively high silicon can also lower the unrecrystallization temperature. Therefore, during the finish rolling in the conventional austenite region, the deformed austenite can complete the dynamic recrystallization process, which is beneficial for obtaining austenite crystal grains with small differences in transverse and longitudinal properties. In the subsequent cooling phase transformation process, martensite and retained austenite with a uniform structure can be obtained. In the composition design, a relatively high manganese content stabilizes austenite. On the other hand, molybdenum significantly delays the transformation of ferrite and pearlite. Martensite imparts high strength to the steel plate, retained austenite imparts high plasticity and cold bending properties to the steel plate, and a uniform and fine structure imparts high hole expansion performance and excellent low-temperature impact toughness to the steel plate.

[0038] In the design of the rolling process, at the stages of rough rolling and finish rolling, the rhythm of the rolling process needs to be completed as soon as possible. After finish rolling, air cooling is first carried out for a certain period of time. The main purposes of air cooling are as follows. Since the contents of manganese and molybdenum are relatively high in the composition design, manganese is an element that stabilizes austenite, and molybdenum significantly delays the phase transformation of ferrite and pearlite. Therefore, during the process of air cooling for a certain period of time, the rolled deformed austenite does not undergo a phase transformation, that is, without forming a ferrite structure, a dynamic recrystallization and relaxation process occur. Due to the dynamic recrystallization of the deformed austenite, nearly equiaxed austenite with a uniform structure can be formed, and the dislocations within the austenite crystal grains are significantly reduced after relaxation. By the combination of both, martensite with a uniform structure can be obtained during the subsequent water quenching process. In order to obtain a martensite structure, the water cooling rate must be greater than the critical cooling rate of low-carbon martensite. In the present invention, the critical cooling rate of martensite is 30 - 50 °C / s according to the composition and process. In order to ensure that all composition designs can obtain martensite, it is necessary to set the water cooling rate of the steel strip to 50 °C / s or higher.

[0039] Since the microstructure according to the present invention is low-carbon or extra-low-carbon martensite, after finish rolling, the steel strip may be cooled to a temperature below the martensite transformation start point Ms at a cooling rate greater than the critical cooling rate. When the cooling stop temperature is different, the amount of retained austenite at room temperature is different. Usually, there is an optimal quenching and cooling stop temperature range, which varies depending on the alloy composition, but is usually 150 to 350°C. In order to obtain high-strength steel with good plasticity and hole expansion rate, it is necessary to rapidly cool the steel strip to a specific temperature range below the Ms point. According to theoretical calculations and actual test verification, when the steel strip is quenched to a range below Ms, a microstructure with excellent comprehensive performance can be obtained. When the quenching temperature is above Ms, although the amount of retained austenite is large, bainite structure appears in the microstructure, and the strength requirement of 980 MPa or more cannot be satisfied. For the above reasons, the coiling temperature needs to be controlled below Ms. The present invention can obtain 980 MPa grade high hole expansion steel with excellent strength, plasticity, toughness, cold bending and hole expansion performance, precisely based on this innovative composition and process design idea.

[0040] The present invention has the following beneficial effects. The present invention adopts a relatively economical composition design idea and an innovative cooling process route to obtain 980 MPa grade high hole expansion steel with excellent strength, plasticity, toughness, cold bending and hole expansion performance.

[0041] The 980 MPa grade steel coil or steel plate described in the present invention has an excellent matching of strength, plasticity and toughness, and also has good cold bending performance and hole expansion and flanging processing performance. It is a hot-rolled or pickled high hole expansion steel with a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, and a thickness of 2 to 6 mm. At the same time, it has good elongation (A in the transverse direction 50 ≧10%) and hole expansion performance (hole expansion rate ≧ 80%), passes the cold bending performance test (d ≦ 4a, 180°), and can be applied to the manufacture of parts such as automobile chassis and subframes that require high strength, thin thickness, and hole expansion and flanging processing, and has a very broad application prospect.

Brief Description of the Drawings

[0042]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0043] Referring to FIGS. 1 to 3, a method for manufacturing a 980 MPa grade ultra-low carbon martensite and retained austenite type extra-high hole-expanding steel according to 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, followed by secondary refining in a vacuum furnace, and then casting into billets or ingots; 2) Reheating the billets or ingots, with the heating temperature being 1100 - 1200 °C and the holding time being 1 - 2 hours; 3) Hot rolling The rolling start temperature was 950 - 1100 °C, and rough rolling was carried out at 950 °C or higher for 3 - 5 passes so that the cumulative deformation amount was 50% or more. Then, the intermediate billets were held at 900 - 950 °C, and finish rolling was further carried out for 3 - 5 passes so that the cumulative deformation amount was 70% or more; the finish rolling temperature was 800 - 920 °C; Path Path 4) Cooling First, air cooling was carried out for 0 - 10 seconds, and then the steel strip was water-cooled to below the martensite transformation start point Ms at a cooling rate of 50 °C / s or more and coiled. After coiling, it was cooled to room temperature (cooling rate ≤ 20 °C / h); 5) Pickling Adjust the pickling running speed of the steel strip within the range of 30 - 100 m / min, control the pickling temperature between 75 - 85 °C, control the tension correction rate within 2% or less, perform rinsing in the temperature range of 35 - 50 °C, dry the surface of the steel strip between 120 - 140 °C, and apply oil.

[0044] The composition of the examples of the high-hole-expanded 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 them, in the rolling process, the thickness of the steel billet is 120 mm; Table 4 is the mechanical properties of the steel plates of the examples of the present invention. Tensile properties (yield strength, tensile strength, elongation rate) were tested according to the ISO6892 - 2 - 2018 international standard, the hole expansion rate was tested according to the ISO16630 - 2017 international standard, and the -40 °C impact toughness was carried out according to the ISO14556 - 2015 international standard. The cold bending performance was carried out according to the ISO7438 - 2005 international standard.

[0045] From Table 4, it can be seen that the yield strength of the steel coils is all 800 MPa or more, the tensile strength is 980 MPa or more, the elongation rate is usually 10 - 13%, the impact energy is relatively stable, the low-temperature impact energy at -40 °C is stable at 140 - 180 J, the retained austenite content changes with the coiling temperature, generally varying between 2 - 5%, and the hole expansion rate satisfies 80% or more.

[0046] From the above examples, the 980 MPa high-strength steel according to the present invention has a good matching of strength, plasticity, toughness and hole expandability, is particularly suitable for high-strength and thin-walled parts such as the chassis structure of automobiles and parts (such as control arms, etc.) that require hole expansion and flange forming, 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.

[0047]

Table 1

[0048]

Table 2

[0049]

Table 3

[0050]

Table 4

Claims

1. A 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet, in which the weight percentages of chemical components are: C: 0.03% to 0.06%, Si: 0.8% to 2.0%, Mn: 1.0% to 2.0%, P: ≤0.02%, S: ≤0.003%, Al: 0.02 to 0.08%, N: ≤0.004%, Mo: 0.1% to 0.5%, Ti: 0.01% to 0.05%, O: ≤0.0030%, and the balance is Fe and other inevitable impurities. The above-mentioned extra-high hole-expanding hot-rolled steel sheet has a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, and a lateral direction A ５０ of 10% or more, and a hole expansion rate of 80% or more. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet has a microstructure of extra-low carbon martensite and retained austenite, and the retained austenite is 5% by volume or less.

2. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, further comprising one or more elements selected from Cr ≤0.5%, B ≤0.002%, Ca ≤0.005%, Nb ≤0.06%, V ≤0.05%, Cu ≤0.5%, and Ni ≤0.5%.

3. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, wherein the weight percentages of chemical components satisfy one or more of the following characteristics: The Cr content is 0.2 to 0.4%; The Cu content is 0.3% or less; The Ni content is 0.3% or less; The Nb content is 0.03% or less; The V content is 0.03% or less; The B content is 0.0005 to 0.0015%; The Ca content is 0.002% or less.

4. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, wherein the weight percentages of chemical components are: C: 0.03% to 0.06%, Si: 0.8% to 2.0%, Mn: 1.0% to 2.0%, P: ≤0.02%, S: ≤0.003%, Al: 0.02 to 0.08%, N: ≤0.004%, Mo: 0.1% to 0.5%, Ti: 0.01% to 0.05%, O: ≤0.0030%, Cr: ≤0.5%, B: ≤0.002%, Ca: ≤0.005%, Nb: ≤0.06%, V: ≤0.05%, Cu: ≤0.5%, Ni: ≤0.5%, and at least one of Cr, B, Ca, Nb, V, Cu, and Ni, and the balance is Fe and other inevitable impurities.

5. A 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet containing Cr and / or B, as described in Claim 4.

6. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the C content is 0.04 to 0.055%.

7. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the Si content is 1.2 to 1.6%.

8. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the Mn content is 1.4 to 1.8%.

9. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the S content is controlled to 0.0015% or less and / or the N content is controlled to 0.003% or less.

10. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the Al content is 0.02 to 0.05%.

11. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the Ti content is 0.01 to 0.03% and / or the Mo content is 0.15 to 0.35%.

12. The 980 MPa grade extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to Claim 1, characterized in that the extra-high hole-expanding hot-rolled steel sheet has an impact toughness of 140 J or more at -40°C.

13. The above-mentioned extra-high hole-expanding hot-rolled steel sheet has a yield strength of 815 MPa or more, a tensile strength of 1000 MPa or more, and a transverse direction A ５０ is 10% or more, the hole-expanding rate is 85% or more, and the -40°C impact toughness is 150 J or more. The 980 MPa extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to claim 1, characterized by the above.

14. A method for manufacturing a 980 MPa extra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to any one of Claims 1 to 13, comprising the following steps: 1) Smelting and casting Smelt according to the composition described in Claims 1 to 11 by adopting a converter or an electric furnace, subject to secondary refining in a vacuum furnace, and then cast into billets or ingots; 2) Reheat the billets or ingots, with the heating temperature being 1100 to 1200°C and the holding time being 1 to 2 hours; 3) Hot rolling The starting temperature of rolling is set at 950 - 1100°C, rough rolling is carried out at 950°C or higher for 3 - 5 passes so that the cumulative deformation amount reaches 50% or more. Then, the intermediate billet is held at 900 - 950°C, and finish rolling is carried out for 3 - 5 passes so that the cumulative deformation amount reaches 70% or more; the finish rolling temperature is set at 800 - 920°C; 4) Cooling First, air cooling is carried out for 0 - 10 seconds, then the steel strip is water-cooled below the martensite transformation start point Ms at a cooling rate of 50°C / s or more and wound up. After winding up, it is cooled to room temperature; 5) Pickling The pickling running speed of the steel strip is adjusted within the range of 30 - 100 m / min, the pickling temperature is controlled between 75 - 85°C, the elongation rate of the tension leveling process is controlled to be 2% or less, then rinsing is carried out, the surface of the steel strip is dried, and oil is applied.

15. The method for manufacturing a 980 MPa ultra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to claim 14, characterized in that in step 5), after pickling, rinsing is carried out in the temperature range of 35 - 50°C, and the surface of the steel strip is dried at 120 - 140°C, and oil is applied.

16. The method for manufacturing a 980 MPa ultra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to claim 14, wherein in step 3), the cumulative deformation amount of the 3 - 5 passes of finish rolling is 85% or more.

17. The method for manufacturing a 980 MPa ultra-low carbon martensite and retained austenite type extra-high hole-expanding hot-rolled steel sheet according to claim 14, wherein in step 4), the cooling rate of water cooling is 50 - 85°C / s.

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